Scenario

A 35 year old woman develops Graves’ hyperthyroidism (the commonest cause of hyperthyroidism) four months after the birth of her second child. She receives treatment with antithyroid drugs for six months. In her third pregnancy she complains of palpitations, excessive sweating, and heat intolerance at 16 weeks’ gestation. Although she experienced these symptoms in previous pregnancies, the current symptoms are much worse.

She is found to be severely hyperthyroid, with raised concentrations of serum free thyroxine (51.7 pmol/l (normal range 9.8-23.1 pmol/l) and free triiodothyronine (19.9 pmol/l (3.5-6.5 pmol/l)) and with suppressed concentrations of thyrotrophin (thyroid stimulating hormone) (<0.02 mU/l (0.35-5.5 mU/l)). She is treated with propylthiouracil, initially 150 mg three times daily, which is reduced eventually to 50 mg twice daily as she becomes euthyroid. Thyrotrophin receptor antibodies are measured at 30 weeks’ gestation and are negative. Propylthiouracil is continued throughout pregnancy and she breast feeds while taking the drug. The drug is stopped two months postpartum; thyroid function is normal three weeks later.