Scholarly article on topic 'Maturation in Serum Thyroid Function Parameters Over Childhood and Puberty: Results of a Longitudinal Study'

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Academic research paper on topic "Maturation in Serum Thyroid Function Parameters Over Childhood and Puberty: Results of a Longitudinal Study"

Maturation in serum thyroid function parameters over childhood and puberty: results of a longitudinal study

Peter N Taylor, Adrian Sayers, Onyebuchi Okosieme, Gautam Das, Mohd S Draman, Arshiya Tabasum, Hussam Abusahmin, Mohammad Rahman, Kirsty Stevenson Alix Groom, Kate Northstone, Wolf Woltersdorf, Andrew Taylor, Susan Ring, John H Lazarus, John W Gregory, Aled Rees, Nicholas Timpson, Colin M Dayan

The Journal of Clinical Endocrinology & Metabolism Endocrine Society

Submitted: November 03, 2016 Accepted: April 24, 2017 First Online: May 01, 2017

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Maturation in serum thyroid function parameters over childhood and puberty: results of a longitudinal study

Peter N Taylor1,2, Adrian Sayers2,3, Onyebuchi Okosieme1,4, Gautam Das4, Mohd S Draman1, Arshiya Tabasum5, Hussam Abusahmin5, Mohammad Rahman5, Kirsty Stevenson6 Alix Groom2,7, Kate Northstone2, Wolf Woltersdorf6,8, Andrew Taylor9, Susan Ring2,7, John H Lazarus1, John W Gregory1, Aled Rees5,10, Nicholas Timpson7, Colin M Dayan1

1) Thyroid Research Group, Systems Immunity Research Institute, Cardiff University School of Medicine, Cardiff UK

Department of Social and Community Medicine, University of Bristol, Bristol, UK

3) Musculoskeletal Research Unit, University of Bristol, Learning & Research, Southmead Hospital, Westbury on Trym, Bristol, UK

4) Endocrinology and Diabetes Department, Prince Charles Hospital, Cwm Taf University Health Board, Merthyr Tydfil, UK

Endocrinology and Diabetes Department, University Hospital of Wales, Cardiff, UK

6) Department of Biochemistry, Bristol Royal Infirmary University Hospitals Bristol NHS Foundation Trust, Bristol UK

MRC Integrative Epidemiology Unit, University of Bristol, Bristol, United Kingdom

8) Facharzt für Laboratoriumsmedizin Geschäftsleiter MVZ Labor Dr. Reising-Ackermann & Kollegen Strümpellstrasse 40 04289 Leipzig, Germany

Department of Biochemistry, Royal United Hospital, Bath, UK

10) Neuroscience and Mental Health Research, Cardiff University School of Medicine, Cardiff, UK Received 03 November 2016. Accepted 24 April 2017.

Context

Serum thyroid hormone levels differ between children and adults, but have not been studied

longitudinally in the same individuals through childhood.

Objective

To assess changes in thyroid stimulating hormone (TSH) and thyroid hormone levels over childhood and their inter-relationships Design Cohort study

Setting

The Avon Longitudinal Study of Parents and Children a population-based birth cohort. Participants

4,442 children who had thyroid function measured at age 7, and 1,263 children who had thyroid function measured at age 15. 884 children had measurements at both ages. Main Outcome Measures

Reference ranges for TSH, free tri-iodothyronine, free thyroxine, their longitudinal stability

and the relationships between them.

Results

Children at age 7 years had a higher FT3 (6.17 pmol/l, SD 0.62) than children at age 15 (5.83 pmol/l, SD 0.74) p <0.0001 with 23.2% of children at age 7 having FT3 above the adult reference-range. Higher FT3 levels at age 7 in boys (p=0.0001) and girls (p=0.04) were associated with attainment of a more advanced pubertal stage at age 13. TSH was positively associated with FT3 at age 7 and age 15 even after adjusting for confounders. In contrast TSH was negatively associated with FT4 at both ages.

Conclusions

There are substantial changes in TSH and thyroid hormone levels over childhood and their relationships, in particular for FT3 which appear to relate to pubertal readiness. Our data provide increased insight into the evolution of the pituitary-thyroid axis over childhood and may have implications for determining optimal ranges for thyroid hormone replacement in children.

We studied thyroid function in children at ages 7 and 15 years. We identified FT3 levels were substantially higher at age 7 and higher levels of FT3 were associated with more advanced puberty.

Introduction

Thyroid hormones play an important role in developmental processes including growth, maintenance of metabolic balance and cell development (1). Even minor variation in thyroid hormone status within the normal population reference-range is associated with important phenotypic consequences (2). The complex inverse relationship between thyroid stimulating hormone (TSH) and free thyroxine (FT4) renders TSH the more sensitive marker of overall thyroid status (3). Free tri-iodothyronine (FT3) is the active thyroid hormone, though serum levels only indirectly reflect overall thyroid status since a substantial proportion of intracellular FT3 is produced from conversion of intracellular FT4 by deiodinases (4,5). However, there is some evidence that T3 may have a more important role than previously assumed in both the assessment and therapy of thyroid disease in younger children (6).

Thyroid hormone levels are largely genetically determined, (7) with similar effects from genetic variation observed in children and adults (8). Whilst it is well established in adults that there is narrow intra-individual variation in thyroid hormone parameters compared to inter-individual variation (9) increased variance and ranges in thyroid hormone levels has been observed throughout childhood and adult reference intervals may not be universally applicable to children (10-12). Previous cross-sectional studies have indicated that free triiodothyronine (FT3) substantially falls and FT4 rises from age 4 (13-15) but there have been no longitudinal studies to confirm these observations. Furthermore from genetic analyses we have recently identified that higher body mass index and adiposity appear to causally increase FT3, but not TSH or FT4 levels (16), therefore the longitudinal stability of thyroid hormones over childhood, and FT3 in particular remains unclear.

In this report, we studied TSH and thyroid hormone levels at ages 7 and 15 in a large population birth cohort. We assessed age and sex reference-ranges in 4,442 healthy children at age 7 and 1,253 children at age 15 (884 children had thyroid function measured at both time points). We also explored the longitudinal variability of TSH and thyroid hormone levels using linear mixed models by sex, pubertal status and BMI and also assessed the relationship between TSH and thyroid hormone at different time-points over childhood.

Methods

Participants

ALSPAC is a prospective birth cohort that enrolled over 13,000 pregnant women in the former County of Avon, UK, with an expected delivery date between April 1991 and December 1992 (17,18) (see www.alspac.bris.ac.uk.) Children were regularly brought back to focus clinics where data were collected and phenotypic measurements and blood samples was taken. The study website contains details of all the data that are available through a fully searchable database www.bris.ac.uk/alspac/researchers/data-access/data-dictionary/. Ethical approval for the study was obtained from the ALSPAC Ethics and Law Committee and the Local Research Ethics Committees. There were no children on levothyroxine or anti-thyroid medications in the study dataset.

Laboratory Measures

TSH, FT3 and FT4 were measured during 2010-2011 on remaining frozen stored serum samples taken from the focus at age 7 years (median age 89 months) and focus at age 15 clinics (median age 184 months). Samples were analyzed using chemiluminescent emission using a photomultiplier on cobas® e601 (Roche Diagnostics, Mannheim Germany). 4,442 samples were available for full thyroid function testing at age 7 years and 1,253 were available at age 15 years. In 884 children samples were available and processed at both ages 7 and 15. Reference-ranges for adults are TSH, 0.27-4.2 mU/liter, FT3 3.9-6.7pmol/liter, FT4 1222 pmol/liter. It has been previously demonstrated that TSH and FT4 can be analyzed reliably in samples stored for up to 23 years (19). The intra-assay precision coefficient of variance for TSH, FT3 and FT4 were <3.1%, < 4% and < 4% respectively. The inter-assay precision coefficient of variance were < 7.3%, <6% and <7% respectively.

Phenotypic measures

Standing height was measured using a wall-mounted Harpenden stadiometer (Holtain Ltd., Crymych, UK). BMI was calculated as weight (in kilograms) divided by height (in meters) squared. Pubertal status was self-assessed using a Tanner stage questionnaire at age 13.5 years (pubic hair domain) range 13.1 to 14.4 years.

Statistical Analysis

Implausible TSH and thyroid hormone levels (> 4 SD from the mean for the sex and age-specific category) were considered as outliers and were recoded to missing. TSH was loge transformed to an approximately normal distribution. Descriptive statistics are presented as geometric means, standard deviations (SD), median and 95th centiles.

A linear mixed model with random intercepts and random slopes was used to assess the trends of TSH and thyroid hormone parameters over childhood (20). An unstructured variance-covariance matrix was assumed. We analyzed the baseline values at age 7, the variability at baseline, the longitudinal trend (slope) between age 7 and 15 and the variability in the slope. Analyses were performed with gender interactions and gender X puberty interactions. Model simplification was undertaken using likelihood ratio tests. Additional analysis was undertaken adjusting for BMI as this may be associated with pubertal development and FT3 in particular or on the causal pathway between thyroid status and pubertal development.

We then explored the relationship between TSH and thyroid hormone levels at ages 7 and 15. Here thyroid function was standardized and therefore results are presented as per SD change in the outcome. Analyses were initially performed adjusted for age at thyroid measurements and gender (model 1). Three further models controlling for key potential confounders were undertaken; model 2 also adjusted for thyroid hormone parameters, model 3 also adjusted for measures of social class and early life environment including parents' home ownership, maternal age at birth of child, maternal highest educational qualification, maternal smoking in pregnancy, family adversity index and parents and home score. Likelihood ratio tests were used to identify if there was any evidence of interaction by sex on the relationship between thyroid hormone parameters and TSH.

Results

Study population and baseline characteristics

The derivation of study participant numbers is shown in Figure 1. A total of 80 children at age 7 (1.8%) and 38 children at age 15 (2.9%) met the outlier exclusion. Children in our final analysis dataset were more likely to have several higher markers of affluence and fewer early life events than the remainder of the ALSPAC cohort (Supplementary Table 1).

Serum thyroid hormone levels in children at ages 7 and 15

At age 7 years the mean and 95% reference range values for TSH, FT3 and FT4 were 2.26 (0.93 - 4.48) mU/l 6.29 (5.13 - 7.59) pmol/l and 15.7 (12.7 - 19.3) pmol/l respectively (Table 1). 23.2% of children at age 7 years had a FT3 above the adult reference range, with only 3.65% of children having a TSH and 0.2% of children having FT4 values above the adult reference-range (Table 1 Figure 2). At age 15 years the mean and 95% reference range values for TSH, FT3 and FT4 were 2.43 (0.91 - 5.05) mU/l 5.83 (4.45 - 7.35) pmol/l and 15.5 (11.9 - 20.3) pmol/l respectively (Table 1 Figure 3), with a marked reduction in children having FT3 above the adult reference-range to 12.2% which was mainly in girls (Table 1). Analysis of just the 884 children who had thyroid function at both age 7 and 15 revealed similar results (Supplementary Table 2). There was a modest correlation between TSH levels between age 7 and 15 (Pearson's correlation coefficient = 0.35), which was similar for FT4 (Pearson's correlation coefficient = 0.33 although a much weaker correlation was observed for FT3 (Pearson's correlation coefficient = 0.10). Bland-Altman plots revealed no evidence of heteroskedasticity for TSH, FT3 and FT4 (Supplementary Figure 1).

Linear mixed models analysis in children with thyroid function at age 7 and age 15 TSH levels rose between ages 7 and 15 years whereas both FT3 and FT4 levels fell. Strong negative correlations were observed in the models for TSH FT3 and FT4 indicating that those with higher levels at age 7 years were more likely to have more substantial lowering of levels at age 15, and those with lower levels at age 7 were likely to have smaller reductions at age 15, i.e. a convergence of biomarkers (Table 2). Every 2 years between ages 7 and 15 years, TSH levels increased by 0.03 mU/l (95%CI 0.02, 0.05) p <0.001. Boys had a higher baseline TSH than girls at age 7 years by 0.11mU/l (95%CI 0.06, 0.17) p <0.001. There was no difference in mean gain between boys and girls between ages 7 and 15 years B =0.0001 (95%CI -0.001, 0.001) p=0.83 and no difference in variability at baseline -0.04 (95%CI -0.10, 0.03) p=0.29 or in the variability of the slope B = 5.73x10-06 (95%CI -0.0002 0.0003) p=0.65 (Table 2).

For FT3 every 2 years between the ages of 7 and 15 years, FT3 levels fell 0.12 pmol/l (95%CI -0.13, -0.10). Girls had a higher baseline FT3 level than boys by 0.13pmol/l (95%CI 0.09, 0.17) p <0.001. However, boys had a reduced decline in FT3 than girls B=0.008 (95%CI 0.007, 0.009) p<0.001. There was no substantial difference by gender in variability at baseline B=0.02 (95%CI -0.01, 0.05) p=0.29, or in variability in slope B = 7.85x10"06 (95%CI -5.18x10-06, 2.01x10"°5) p=0.24 (Table 2). Every 2 years FT4 levels fell 0.04 pmol/l (95%CI -0.07, -0.01) p=0.005. Girls had a higher baseline FT4 level than boys by 0.38pmol/l (95%CI 0.28, 0.48) p<0.001 and also had more variability at baseline at age 7 years B =0.38 (95%CI 0.14, 0.62) p=0.002 although there was no difference in variability in slope B =4.47x10"05. (95%CI 4.47x10-05, 0.001) p=0.33 (Table 2). Adjusting the analysis for BMI revealed similar results, although it markedly attenuated the slope for TSH (Supplementary Table 3).

Relationship between pubertal status at age 13 and TSH and thyroid hormone parameters at aged 7 and 15

2,702 children also had pubertal status self-assessed at age 13 years as well as having thyroid function measured. As expected, girls had a higher Tanner score than boys 3.63 (95%CI 3.58, 3.69) vs 2.96 (95%CI 2.89, 3.02) p<0.0001. Pubertal status at age 13 years was not associated with TSH levels at age 7 in boys (p=0.89) or girls (p=0.31). No difference in TSH slope by pubertal status was observed in boys (p=0.82) or girls (p=0.82). Pubertal status at age 13 years was also not associated with FT4 levels at age 7 years in boys (p=0.32) or girls (p=0.52). By contrast, FT3 levels at age 7 years were higher in both boys (p=0.0001) and girls (p=0.04) with more advanced puberty at age 13 years (Table 3) More advanced pubertal status at age 13 years was however associated with a negative FT3 slope unlike children at an earlier pubertal status at age 13 which had a positive FT3 slope, in both boys and girls

(p=<0.001). Similarly, there was no evidence of any difference in the variability of baseline values or gradients of slopes by pubertal status in either boys or girls for either FT3 or FT4. Although BMI at age 7 was also associated with Tanner stage at age 13 B=0.08 (95%CI 0.06, 0.09) p<0.001 and FT3 B=0.04 (95%CI 0.03, 0.05) p<0.001 adjusting for BMI at age 7 had no substantial effect the relationship between FT3 and Tanner stage. Analysis of the association between FT3 and Tanner stage when adjusted for sex was B=0.12 (95%CI 0.07, 0.12) p<0.001 adding BMI to the model had a minimal impact on effect estimates B=0.10 (95%CI 0.05, 0.15) p<0.001. Furthermore, adjustment for BMI in the linear mixed models performed by pubertal status revealed very similar results to our original analysis (Supplementary Table 4).

Relationship between TSH and serum thyroid hormone levels in children at ages 7 and 15 years At age 7 years, TSH was weakly positively associated with FT3 after adjusting for age, sex, FT4 and markers of social class and early life environment B (std) =0.03 (95%CI 0.001, 0.06) p=0.05 whereas TSH was clearly negatively associated with FT4 B (std) =-0.07 (95%CI -0.10, -0.04) p=3.49x10-05 (Supplementary Table 5). A similar pattern was also observed at age 15 years even after adjusting for pubertal status, with TSH positively associated with FT3 B (std) =0.07 (95%CI 0.02, 0.13) p=0.01 and negatively associated with FT4 B (std) =-0.13 (95%CI -0.19, -0.07) p=5.16x10-06 (Supplementary Table 5). FT3 and FT4 were positively associated with each other at age 7 years B (std) =0.27 (95%CI 0.24, 0.30) p=1.12x10-14 and also at age 15 years, B =0.19 (95%CI 0.12, 0.26) p=4.23x10-07. Seemingly unrelated regression identified that the positive impact of TSH on FT3 was greater at age 15 years than at age 7 years (p=0.001), but no difference was observed with FT4 (p=0.84).

Discussion

Our results from a longitudinal analysis of a large population birth cohort demonstrate that there are substantial changes in the pituitary-thyroid axis over childhood. In particular, FT3 changes much more over childhood than either TSH or FT4. Levels of FT3 at age 7 are high compared to adult values with almost 25% of children at age 7 years having a FT3 level above the adult reference-range. Although there is a substantial fall in FT3 levels between age 7 years and age 15 years, 10% are still above the adult reference-range.

There was a very strong negative correlation between hormone levels between age 7 and 15, indicating that the substantial variability observed in early childhood is reduced through puberty, with hormone levels converging to near adult reference values. Overall our data suggests that there may be higher conversion of FT4 to FT3 in younger children than adults. Our observation that boys maintain a higher FT3 for longer than girls is also noteworthy, and may have substantial importance in observed sex differences in bone development (21) and other phenotypes (2).

The reason why children have higher FT3 levels at age 7 years is unclear but may be due to factors external to the pituitary-thyroid axis such as fat mass and pubertal development (16). In the current study, we noted that children that reached puberty earlier (as indicated by more advanced self-reported pubertal stage at age 13 years) had higher FT3 values at age 7 years and a negative FT3 slope between ages 7 and 15 years whereas those with less advanced puberty had a positive FT3 slope between ages 7 and 15 years. Using both serum thyroid function and then genetic data to perform Mendelian Randomization we have recently reported that BMI and fat mass in children is positively and causally related to FT3 (16). Although the effect of FT3 on puberty is interestingly largely independent of BMI, it is however still possible that FT3 is an indicator of nutritional state and hence pubertal "readiness" in early childhood in a manner similar to leptin. Alternatively, the observed changes may represent changes in the thyroid gland in preparation for puberty, or be a

consequence of changes in other endocrine factors such as growth hormone, as growth hormone therapy has been linked to marginally increased FT3 and decreased FT4 levels (22).

We have also identified a difference in the relationship between TSH and the two thyroid hormones FT3 and FT4 in childhood with higher TSH being associated with higher FT3 whereas an inverse association was identified with FT4. The positive association between TSH and FT3 in childhood has been highlighted recently in children with borderline thyroid status (23). This observation provides new insight into childhood TSH-FT4 and TSH-FT3 relationships which are relevant to our understanding of both thyroid physiology and the laboratory diagnosis of thyroid disease. It is interesting to speculate the life course of FT3 levels given it is well established that FT3 in particular declines in the elderly; (24) the pattern of FT3 through life may therefore be a fall over childhood (25), then plateauing throughout adult life, before falling again in older age.

We believe our findings are also clinically relevant, given the striking differences observed in early childhood thyroid hormone levels from adult derived reference-ranges. If age and sex appropriate reference ranges are not used, there may be substantial over-diagnosis of sub-clinical thyroid disease in children. In addition, our finding that children have substantially higher FT3 levels than adults may have implications for thyroid hormone replacement in children. Individuals on levothyroxine have a higher FT4 and a lower FT3 than euthyroid individuals despite having similar TSH levels (26-28). Children on levothyroxine might therefore have inadequate FT3 levels for optimal timing of puberty and other developmental processes. It is noteworthy that hypothyroidism diagnosed in pre-pubertal years, can cause a delay of puberty (29) It is also possible that the relative lack of FT3 in these children may potentially be one of the reasons that optimal IQ levels are not reached in children with congenital hypothyroidism despite adequate levothyroxine therapy (30). Taken together, there remains a pressing need for further study of central and peripheral determinants of thyroid function as well as determinants of intracellular thyroid status in children.

Strengths of our dataset include the use of a large population birth cohort with detailed phenotypic data available and paired thyroid function at two age-points which allows more robust analysis than previous studies of cross-sectional samples. The nature of the cohort means it is unlikely that interfering medications or heterophilic antibodies have influenced results. Furthermore, our use of liner mixed models has allowed us to determine the change of TSH and thyroid hormone levels between ages 7 and 15, whilst simultaneously adjusting for an individual's baseline hormone levels, allowing us to investigate how variability reduces as children progress into adulthood. Limitations of our study include a higher social class bias in our dataset and lack of generalizability to ethnic minorities, as 98% of all samples analyzed were in individuals of Caucasian descent. A weakness is that paired samples were also not all performed on the same assay run. Furthermore, all individuals were from a small region of the UK which has been shown to be borderline iodine deficient (31). Our findings require replication in individuals from other ethnic groups and using different thyroid hormone assays from an area of iodine sufficiency.

In conclusion, our results demonstrate that thyroid hormone levels change substantially during childhood and adolescence. This is particularly the case with FT3, which is substantially higher in younger children. FT3 levels also appear to influence the onset of puberty; further studies into the pituitary-thyroid axis in normal childhood populations are therefore needed to define the role of higher FT3 levels in childhood more precisely.

Acknowledgements

We are extremely grateful to all the families who took part in this study, the midwives for their help in recruiting them, and the whole ALSPAC team, which includes interviewers,

computer and laboratory technicians, clerical workers, research scientists, volunteers, managers, receptionists and nurses. The UK Medical Research Council and Wellcome (Grant ref: 102215/2/13/2) and the University of Bristol provide core support for ALSPAC. This publication is the work of the authors and Peter Taylor and Colin Dayan will serve as guarantors for the contents of this paper. Thyroid function was performed in ALSPAC using grants from the BUPA research foundation, British Thyroid Association and the Above and Beyond foundation.

Name and Address of corresponding author: Peter Taylor, Thyroid Research Group, Systems Immunity Research Institute Medicine, C2 link corridor, UHW, Cardiff University School of Medicine, Heath Park email: taylorpn@cardiff. ac.uk telephone: 00447590520741 fax: 0044 29 20 744671

Please send address for re-prints to taylorpn@cardiff.ac.uk

DISCLOSURE STATEMENT: The authors have nothing to disclose.

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Figure 1

Figure 2A

Figure 2B

Figure 2C

Figure 3A range)

Study participants

Histogram of TSH levels at age 7 (vertical lines refer to adult reference-range) Histogram of FT3 levels at age 7 (vertical lines refer to adult reference-range) Histogram of FT4 levels at age 7 (vertical lines refer to adult reference-range) Histogram of TSH levels at age 15 (vertical lines refer to adult reference-

Figure 3B range)

Figure 3C range)

Histogram of FT3 levels at age 15 (vertical lines refer to adult reference-Histogram of FT4 levels at age 15 (vertical lines refer to adult reference-Table 1 Reference-range for thyroid hormone parameters age 7 and age 15

Age (year s) All Males Females

N Mea n (2.597.5 %) % abo ve AR R % Belo w ARR N Mea n (2.597.5 %) % abo ve AR R % belo w AR R N Mea n (2.597.5 %) % abo ve RR % belo w RR

TSH (mU/l ) 4,44 2 2.26 0.93 -4.48 3.65 0 2,32 3 2.32 0.97 - 4.50 3.57 0 2,11 9 2.20 0.88 4.45 3.73 0

FT3 (pmol /l) 7 4,44 2 6.29 5.13 7.59 23.2 0.09 2,32 3 6.23 5.07 7.56 19.8 0.17 2,11 9 6.35 5.167.59 26.9 0

FT4 (pmol 4,42 2 15.7 12.7 0.20 0.70 2,32 3 15.6 12.7 0.17 0.73 2,11 9 15.9 12.85 0.24 0.66

/l) 19.3 19.0 19.55

TSH (mU/l ) 1,26 3 2.43 0.91 5.05 6.33 0 644 2.51 0.915.17 7.92 0 619 2.34 0.87 5.00 4.68 0

FT3 (pmol /l) 15 1,26 3 5.83 4.45 7.35 12.2 0.55 644 6.16 4.84 - 7.6 20.7 0 619 5.48 4.23 6.91 3.39 1.13

FT4 (pmol /l) 1,26 3 15.5 11.9 20.3 0.79 2.69 644 15.5 11.8 20.2 0.62 2.95 619 15.5 12.0 20.6 0.97 2.42

N=Number

ARR= Adult reference range

Table 2 Overall linear mixed models for TSH FT3 and FT4

Coefficient 95% CI P-Value

TSH (mU/l) All Main effects Age 7 years 2.27 (2.24, 2.3) <0.001

Slope 0.0013 (0.0007, 0.002) <0.001

Variability SD@ Age 7 years 1.62 (1.56 1.67)

SD Slope 0.14 (0.13, 0.14)

Correlation(int,slope) -0.87 (-0.89, -0.86)

FT3 (pmol/l) All Main effects Age 7 years 6.29 (6.27, 6.31) <0.001

Slope -0.005 (-0.005, -0.004) <0.001

Variability SD@ Age 7 years 1.28 (1.24, 1.32)

SDSlope 0.12 (0.11, 0.12)

Correlation(int,slope) -0.92 (-0.93, -0.91)

FT4 (pmol/l) All Main effects Age 7 years 15.7 (15.7, 15.8) <0.001

Slope -0.002 (-0.03, -0.0005) 0.005

Variability SD@ Age 7 3.03 (2.92 3.14)

SDSlope 0.27 (0.26, 0.28)

Correlation(int,slope) -0.86 (-0.88, -0.84)

TSH Boys Main effects Age 7 2.32 2.28 2.36 <0.001

Slope 0.001 0.0005 0.002 0.002

Variability SD@ Age 7 0.9 0.87 0.92

SDSlope 0.01 0.01 0.01

Correlation(int,slope) -0.46 -0.52 -0.39

FT3 Boys Main effects Age 7 6.23 6.2 6.25 <0.001

Slope -0.0005 -0.001 0.0000889 0.09

Variability SD@ Age 7 0.63 0.61 0.64

SDSlope 0.009 0.008 0.009

Correlation(int,slope) -0.58 -0.63 -0.53

FT4 Boys Main effects Age 7 15.5 15.4 15.6 <0.001

Slope 0.0003 -0.001 0.002 0.72

Variability SD@ Age 7 1.63 1.58 1.68

SDSlope 0.022 0.021 0.023

Correlation(int,slope) -0.41 -0.48 -0.34

TSH Girls Main effects Age 7 2.21 2.17 2.24

Slope 0.001 0.0004 0.002 <0.001

Variability SD@ Age 7 0.92 0.89 0.95

SDSlope 0.01 0.01 0.01

Correlation(int,slope) -0.52 -0.58 -0.46

FT3 Girls Main effects Age 7 6.36 6.33 6.38 <0.001

Slope -0.009 -0.01 -0.08 <0.001

Variability SD@ Age 7 0.61 0.59 0.63

SDSlope 0.009 0.008 0.009

Correlation(int,slope) -0.61 -0.66 -0.56

FT4 Girls Main effects Age 7 15.9 15.8 16 <0.001

Slope -0.004 -0.006 -0.002 <0.001

Variability SD@ Age 7 1.74 1.69 1.8

SDSlope 0.022 0.21 0.22

Correlation(int,slope) -0.42 -0.49 -0.35

Table 3 Linear mixed models for TSH FT3 and FT4 by pubertal status at age 13 years

P1 P2 P3

Coeffi cient 95% CI P- Valu e Coeffi cient 95% CI P- Valu e Coeffi cient 95% CI P- Valu e

TS H Bo ys Main effects Age 7 2.37 2.28 2.46 <0.0 01 2.4 2.3 2.5 1 <0.0 01 2.38 2.2 9 2.4 6 <0.0 01

Slope 0.002 0 0.00 4 0.05 0.001 0.0 01 0.0 03 0.19 0.001 0.0 01 0.0 02 0.26

Variab ility SD@ Age 7 0.92 0.85 0.98 0.94 0.8 6 1.0 1 0.94 0.8 8 1.0 1

SDSlope 0.01 0.01 0.01 0.01 0.0 1 0.0 1 0.01 0.0 1 0.0 1

Correlation(i nt,slope) -0.47 0.59 0.34 -0.41 0.5 7 0.2 6 -0.46 0.5 8 0.3 3

T3 Bo ys Main effects Age 7 6.14 6.08 6.2 <0.0 01 6.18 6.1 1 6.2 5 <0.0 01 6.32 6.2 6 6.3 8 <0.0 01

Slope 0.002 0.00 03 0.00 3 0.01 0 0.0 01 0.0 01 0.99 -0.003 0.0 04 0.0 02 <0.0 01

Variab ility SD@ Age 7 0.63 0.59 0.68 0.59 0.5 4 0.6 4 0.63 0.5 8 0.6 7

SDSlope 0.008 0.00 7 0.00 9 0.01 0.0 08 0.0 1 0.009 0.0 08 0.0 1

Correlation(i nt,slope) -0.53 0.64 0.41 -0.66 0.7 7 0.5 6 -0.62 0.7 1 0.5 2

T4 Bo ys Main effects Age 7 15.6 15.4 15.7 <0.0 01 15.6 15. 5 15. 8 <0.0 01 15.5 15. 3 15. 6 <0.0 01

Slope -0.004 0.00 7 0.00 1 0.02 -0.004 0.0 08 0.0 01 0.02 0.005 0.0 02 0.0 09 0.00 1

Variab ility SD@ Age 7 1.58 1.47 1.69 1.6 1.4 6 1.7 3 1.66 1.5 4 1.7 7

SDSlope 0.02 0.02 0.03 0.02 0.0 2 0.0 3 0.02 0.0 2 0.0 3

Correlation(i nt,slope) -0.5 0.62 0.38 -0.5 0.6 4 0.3 6 -0.33 0.4 7 0.1 9

TS H Gi rls Main effects Age 7 2.16 2.04 2.28 <0.0 01 2.28 2.1 7 2.3 9 <0.0 01 2.2 2.1 3 2.2 7 <0.0 01

Slope 0.0004 0.00 1 0.00 2 0.71 0.001 0.0 01 0.0 03 0.21 0.001 0.0 03 0.0 02 0.15

Variab ility SD@ Age 7 0.86 0.76 0.94 0.95 0.8 7 1.0 2 0.92 0.8 8 0.9 7

SDSlope 0.01 0.01 0.01 0.01 0.0 1 0.0 1 0.01 0.0 1 0.0 1

Correlation(i nt,slope) -0.6 0.74 0.45 -0.57 -0.7 0.4 5 -0.52 0.6 1 0.4 4

T3 Gi rls Main effects Age 7 6.27 6.19 6.36 <0.0 01 6.27 6.2 6.3 4 <0.0 01 6.37 6.3 2 6.4 1 <0.0 01

Slope -0.007 0.00 9 0.00 5 <0.0 01 -0.007 0.0 08 0.0 06 <0.0 01 -0.01 0.0 1 0.0 1 <0.0 01

Variab ility SD@ Age 7 0.6 0.54 0.66 0.63 0.5 7 0.6 8 0.62 0.5 9 0.6 5

SDSlope 0.008 0.00 7 0.01 0.008 0.0 06 0.0 09 0.009 0.0 08 0.0 1

Correlation(i nt,slope) -0.77 0.86 0.68 -0.57 -0.7 0.4 4 -0.6 0.6 7 0.5 2

T4 Gi rls Main effects Age 7 15.9 15.6 16.2 <0.0 01 15.8 15. 6 16 <0.0 01 15.9 15. 8 16 <0.0 01

Slope -0.004 0.00 8 0.00 03 0.07 -0.002 0.0 06 0.0 02 0.43 -0.004 0.0 06 0.0 01 0.00 5

Variab SD@ Ago 7— 1 88 1.68 2.06 -179- 1.6 1.9 -137- 1.6 1.8

ility 4 3 8 6

SDSlope 0.02 0.02 0.02 0.02 0.0 2 0.0 3 0.02 0.0 2 0.0 3

Correlation(i nt,slope) -0.42 0.61 0.23 -0.43 0.5 8 0.2 7 -0.43 0.5 3 0.3 3

14,701 children in the ALSPAC cohort alive at 1 year of age.

31 children had an outlier TSH 20 children had an outlier FT3 29 children had an outlier FT4

13,404 children did not have thyroid function available at age 15

28 children had an outlier TSH 2 children had an outlier FT3 8 children had an outiier FT 4

4,442 children had full thyroid function available at age 7. (27 children did not have BMI available)

1,263 children had full thyroid function available at atrc 1

884 children had full (thyroid function available at ages 7 and 15.

FT4 age 7 (pmol/l)

ENDOCRINES! ЛП\/ЛМРР ADTIOI P- 1ГСМ the journal of clinical SOCIETY Si nUVnIMuL МП I IULE. J Vi СIVI endocrinology & metabolism