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Question 489
Endocrinology → General
Classification
Question
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A patient with congenital goitre has low circulating thyroid hormone concentrations, raised TSH and high thyroid uptake of radioisotope. A perchlorate test produces washout of trapped iodide. Which biochemical defect best accounts for this pattern?
Question Stem
Select the single best answer.
Options
A
Failure of peripheral conversion of T4 to T3
B
Failure of TSH secretion
C
Excessive conversion of T4 to reverse T3
D
Failure of iodide trapping by the thyroid
E
Failure of thyroid peroxidase-dependent oxidation and organification of iodide
Explanation
The most common enzyme abnormality in thyroid dyshormonogenesis is absent or insufficient thyroid peroxidase activity. Iodide can be trapped by the thyroid but cannot be adequately oxidised and organified. Reduced T3 and T4 removes negative feedback, increasing TSH and producing goitre. Radioisotope uptake can therefore be high, while a perchlorate washout test is positive because trapped iodide has not been organified.
Option Validity
A) Impaired peripheral T4-to-T3 conversion does not explain failure of iodide organification with a positive perchlorate washout test.
B) TSH is raised in this pattern, not deficient.
C) Increased reverse T3 formation does not account for trapped but non-organified iodide.
D) Iodide trapping is preserved, as demonstrated by high uptake; the defect occurs after trapping.
Further Reading
Saxena, MRCOG-1 Basic Sciences, Endocrinology: Disorders of the Thyroid Gland - dyshormonogenesis, pp. 307–308.