SBA — Single Best Answer · Review
Question 512
Endocrinology → General
Classification
Question
SSBAQuestion Header
A patient with congenital goitre has defective thyroid peroxidase activity. Radioisotope studies show that iodine is taken up by the thyroid but cannot be retained normally after perchlorate administration. Which biochemical step is primarily defective?
Question Stem
Select the single best answer.
Options
A
Peripheral 5'-monodeiodination of T4
B
Binding of circulating T4 to thyroxine-binding globulin
C
Oxidation and organification of trapped iodide
D
Release of TSH from the anterior pituitary
E
Conversion of cholesterol to pregnenolone
Explanation
Thyroid peroxidase deficiency impairs oxidation and organification of iodide. Iodine can therefore be trapped by the thyroid but cannot be incorporated normally into thyroid hormone. Reduced T3 and T4 increase TSH stimulation, producing goitre. A positive perchlorate washout test supports an organification defect.
Option Validity
A) 5'-monodeiodination is the peripheral conversion of T4 to active T3 and is not the thyroid organification defect described.
B) T4 binding to thyroxine-binding globulin occurs in the circulation and does not explain a positive perchlorate washout test.
D) TSH is increased in response to reduced thyroid hormone production; deficient TSH release is not the primary abnormality.
E) Cholesterol-to-pregnenolone conversion is part of steroidogenesis rather than thyroid hormone synthesis.
Further Reading
Saxena, MRCOG-1 Basic Sciences, Endocrinology: Thyroid Dyshormonogenesis.