medwireNews: Levels of thyroid hormones, specifically free tri-iodothyronine (FT3), change markedly during childhood, data from a large prospective study show.

The findings emerge from an analysis of children in the Avon Longitudinal Study of Parents and Children – 4442 with thyroid function tests at age 7 years and 1263 with tests at age 15 years (884 were tested at both times).

At age 7 years, the children’s FT3 levels were high, at an average of 6.29 pmol/L, with 23.2% having a level above the normal adult reference range (3.9–6.7 pmol/L). By age 15 years, the average level had fallen to 5.83 pmol/L and only 12.2% of participants, mostly girls, had levels above the normal adult range.

By contrast, levels of thyroid stimulating hormone (TSH) and free thyroxine (FT4) at age 7 years were similar to adult levels, with just 3.65% and 0.20% of children, respectively, having above-range levels. Average TSH levels rose slightly between ages 7 and 15, from 2.26 to 2.43 mU/L, whereas FT4 levels fell from 15.7 to 15.5 pmol/L.

“We believe our findings are […] clinically relevant, given the striking differences observed in early childhood thyroid hormone levels from adult derived reference-ranges”, write Peter Taylor (Cardiff University School of Medicine, UK) and co-researchers in The Journal of Clinical Endocrinology & Metabolism.

“If age and sex appropriate reference ranges are not used, there may be substantial overdiagnosis of sub-clinical thyroid disease in children.”

Children with more advanced pubertal stage at age 13 years had higher FT3 levels at age 7 years, and also had the largest declines by the age of 15, whereas those who were less advanced had lower levels. FT4 and TSH levels were not associated with pubertal status.

Taylor and team note their findings “may have implications for thyroid hormone replacement in children”, because levothyroxine treatment results in higher FT4 and lower FT3 levels than normal, which in children might result in “inadequate FT3 levels for optimal timing of puberty and other developmental processes.”

This could even explain why children taking levothyroxine fail to achieve their optimal IQ, they suggest.

“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”, the team concludes.

By Eleanor McDermid

J Clin Endocrinol Metab 2017; Advance online publication

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