Mon22November1200AM 14

The thyroid hormones play a crucial role in the control of cellular metabolism and regulation of normal growth and development.

Which of the following thyroid hormones is the single most physiologically active?

(Please select 1 option)

T3 Correct

T4

rT3

T0a

T1a

Explanation

Triiodothyronine (T3) and thyroxine (T4) are the principal thyroid hormones produced (THs), stored and secreted by the thyroid gland.

Thyroid stimulating hormone (TSH) from the anterior pituitary gland stimulates the release of T4 and T3 into the circulation. The ratio T4 to of secreted T3 is in the order of 200:1. Serum T4 is 40-fold higher than T3. However, only 0.03% of T4 is unbound to plasma proteins and therefore unavailable to diffuse into the target cells compared with 0.3% unbound T3. A majority of circulating T3 results from 5'-deiodination of the outer ring of T4 by deiodinases.

Type I deiodinase is found in peripheral tissues such as liver and kidney and is responsible for the conversion of the majority of T4 to T3 in circulation. Type II deiodinase is found in brain, pituitary, and brown adipose tissue and primarily converts T4 to T3 for intracellular use.

Reverse T3 (rT3) is an isomer of T3 and is inactive. It differs from T3 in that the missing deiodinated iodine is from the inner ring of the thyroxine molecule compared with outer ring on T3. In some conditions an excessive production of rT3 can lead to competition for the 5'-deiodinase enzyme system leading to less peripheral conversion of T4 to T3.

The T3 molecules are further processed by decarboxylation and deiodination for the production of iodothyronamine (T1a) and thyronamine (T0a) respectively. These are also inactive metabolites.

Answer Statistics

1

82%

2

10%

3

10%

Times answered: 248