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Question 493
Endocrinology → General
Classification
Question
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During normal pregnancy, rising oestrogen markedly increases thyroxine-binding globulin. Which thyroid-function pattern is expected as a consequence?
Question Stem
Select the single best answer.
Options
A
Reduced total T4 and T3 with unchanged free thyroid hormone
B
Reduced total and free thyroid hormone concentrations
C
Unchanged total T4 and T3 with increased free thyroid hormone
D
Increased total T4 and T3 with sustained elevation of free thyroid hormone
E
Increased total T4 and T3 while free thyroid hormone remains essentially unchanged
Explanation
Oestrogen increases thyroxine-binding globulin during pregnancy. This increases circulating total T4 and T3, while the biologically available free thyroid hormone remains essentially unchanged and the mother remains euthyroid. Free T4 may lie in the low-normal range because of dilutional effects.
Option Validity
A) Total T4 and T3 increase rather than decrease when thyroxine-binding globulin rises.
B) Pregnancy does not normally produce a parallel reduction in total and free thyroid hormones from increased thyroxine-binding globulin.
C) Total T4 and T3 do not remain unchanged; they increase.
D) The rise in total hormone does not imply a sustained elevation of biologically available free hormone.
Further Reading
Saxena, MRCOG-1 Basic Sciences, Endocrinology: Thyroid Gland in Pregnancy, p. 308.