SBA — Single Best Answer · Review
Question 501
Endocrinology → General
Classification
Question
SSBAQuestion Header
Peripheral metabolism of thyroxine can generate either biologically active T3 or largely inactive reverse T3. Which conversion produces active T3?
Question Stem
Select the single best answer.
Options
A
3'-monodeiodination of T4
B
5'-monodeiodination of T4
C
Oxidation of T3 by thyroid peroxidase
D
Conjugation of T4 with glucuronic acid in the thyroid follicle
E
Iodination of reverse T3 in the pituitary
Explanation
T4 is converted to the more biologically active T3 by 5'-monodeiodination in tissues including the liver, kidney and muscle. In contrast, 3'-monodeiodination produces reverse T3, which is largely inactive.
Option Validity
A) 3'-monodeiodination produces largely inactive reverse T3.
C) Thyroid peroxidase participates in thyroid hormone synthesis rather than this peripheral T4-to-T3 conversion.
D) This is not the pathway producing active T3 from T4.
E) Active T3 formation occurs by deiodination rather than iodination of reverse T3 in the pituitary.
Further Reading
Saxena, MRCOG-1 Basic Sciences, Endocrinology: Thyroid Gland - thyroid hormone metabolism, p. 308.