Question 17

Describe the physiology of the thyroid hormones. 

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College Answer

Thyroid hormones consist of thyroxine (T4), tri-iodothyronine (T3) and reverse T3 (rT3). It was 
expected candidates would briefly describe each of these. T4 is a pro-hormone synthesized 
from tyrosine in follicular cells of the thyroid gland and represents 80% of body’s thyroid 
hormone production. It exists in free form, plasma protein bound (albumin and pre-albumin 
(TBPA) and tissue protein bound thyroid-binding globulin (TBG) and has a half-life around 7 
days. Tri-iodothyronine (T3) is the most biologically active thyroid hormone (5 times T4), is 
produced directly from tyrosine (20%) or in the periphery by conversion of T4 (80%) with a 
half-life 1.5 days. Reverse T3 (rT3) is formed via peripheral conversion of T4 by de-iodination.
A classic negative feedback loop exists to control thyroid hormone secretion. Thyroid 
Stimulating Hormone (TSH) from the anterior pituitary is controlled by Thyrotropin Releasing 
Hormone (TRH) from the Hypothalamus via hypothalamic-hypophyseal portal system. Both of 
these factors are inhibited by elevated levels of T4 and T3.
The mechanism of action is by binding to nuclear receptors to effect protein synthesis. Thyroid 
hormone has a wide variety of physiological effects across many systems including respiratory, 
cardiovascular, metabolic and growth and sexual function. 
The answer required candidates to detail both the synthesis and control of thyroid hormones 
as well discussing the action of thyroid hormones. Few candidates could differentiate the roles 
and actions of T3 and T4.