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The thyroid gland and disorders of thyroid function 

The thyroid gland and disorders of thyroid function
Chapter:
The thyroid gland and disorders of thyroid function
Author(s):

Anthony P. Weetman

, and Kristien Boelaert

DOI:
10.1093/med/9780198746690.003.0246
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date: 27 February 2021

The iodine-containing thyroid hormones triiodothyronine (T3) and thyroxine (T4) have diverse effects on metabolism and are essential for normal development, particularly of the fetal brain. The active principle, T3, binds to nuclear receptor isoforms and serves as a transcriptional regulatory factor, thus explaining the protean actions. Thyroid hormone release is regulated by thyrotropin (TSH) from the anterior pituitary, which is itself modulated by the hypothalamic tripeptide, thyrotropin-releasing hormone. A normal TSH level rules out primary thyroid dysfunction, but when TSH levels are abnormal, or when pituitary or hypothalamic abnormalities are possible, it is essential to confirm thyroid status by measuring circulating thyroid hormone levels, which is best achieved by immunoassay of free T3 and free T4. Thyroid-antibody measurement and imaging by scintiscanning are useful in determining the aetiology of thyroid disease when this is not obvious clinically.

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