A 19-year-old with a complete hydatidiform mole at 14 weeks has a palpable thyroid, fine tremor, and suppressed TSH with FT4 above the upper limit of normal. What is the mechanism of her thyrotoxicosis?
- A Autonomous thyroid adenoma secreting T3 and T4
- B TSH receptor antibodies crossing from trophoblastic tissue
- C High concentrations of hCG stimulating the TSH receptor owing to structural homology of the alpha subunit ✓
- D Unique beta subunit of hCG acting directly on thyroid follicles
Correct answer: C. High concentrations of hCG stimulating the TSH receptor owing to structural homology of the alpha subunit
Explanation
hCG and TSH share an identical alpha subunit, and at very high titres, typically above 100,000 IU/L as seen in complete moles, hCG cross-stimulates the TSH receptor causing biochemical and sometimes clinical thyrotoxicosis. It resolves spontaneously after evacuation of the mole. The beta subunit is the hormone-specific portion responsible for hCG's biological uniqueness and does not mediate thyroid stimulation.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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