A 22-year-old woman with a complete hydatidiform mole at 12 weeks has a serum thyroxine level in the thyrotoxic range but no goitre, exophthalmos or thyroid antibodies. What is the underlying mechanism?
- A Autonomous thyroid adenoma secreting T4
- B Cross-reactivity of extremely high hCG concentrations with the TSH receptor ✓
- C Trophoblastic secretion of thyroid stimulating immunoglobulin
- D Peripheral conversion defect causing increased free T4 fraction
Correct answer: B. Cross-reactivity of extremely high hCG concentrations with the TSH receptor
Explanation
hCG shares structural homology with TSH and at the extreme concentrations produced by a complete mole, typically exceeding 100,000 mIU/mL, it directly stimulates the TSH receptor, producing biochemical hyperthyroidism without intrinsic thyroid disease. There is no thyroid autoimmunity or adenoma, and the trophoblast does not secrete thyroid stimulating immunoglobulin. Evacuation of the mole promptly normalises thyroid function, confirming the hormonal drive, so antithyroid drugs are needed only temporarily if symptoms are significant.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.