SBA — Single Best Answer · Review
Question 507
Endocrinology → General
Classification
Question
SSBAQuestion Header
Which property best explains why T4 has a longer biological half-life than T3?
Question Stem
Select the single best answer.
Options
A
Greater protein binding of T4
B
Greater receptor affinity of T4
C
More rapid renal excretion of T3 caused by its larger molecular size
D
Exclusive synthesis of T4 outside the thyroid gland
E
Conversion of T3 to T4 in skeletal muscle
Explanation
T4 has a longer biological half-life than T3 because T4 is more extensively protein bound. In general, a shorter hormone half-life permits more precise and continuous regulation of circulating activity.
Option Validity
B) The longer half-life described is related to greater protein binding rather than receptor affinity.
C) The difference in half-life is attributable to protein binding, not to greater molecular size or this renal mechanism.
D) T4 is produced by the thyroid gland.
E) Peripheral metabolism converts T4 to T3 by 5'-monodeiodination, not T3 to T4 by the mechanism described.
Further Reading
Saxena, MRCOG-1 Basic Sciences, Endocrinology: Half-life of Hormones.