This patient has hyperthyroidism occurring during pregnancy and should be treated with propylthiouracil (PTU). Based on this patient's age and sex, the most likely cause of her hyperthyroidism is Graves disease. Clinical findings supporting this diagnosis are the diffuse thyromegaly, thyroid bruit, and elevated thyroid-stimulating immunoglobulin index. Treatment of hyperthyroidism during pregnancy is typically with medical management, and PTU, rather than methimazole, is the drug of choice during the first trimester. Although rare, methimazole has been associated with development of aplasia cutis (absence of a portion of skin on the scalp in a localized or widespread area) and choanal atresia (blockage of the posterior nasal passage due to failed recanalization of the nasal fossae during fetal development) when used in the first trimester. Once organogenesis is complete, methimazole can be substituted for PTU. Methimazole is typically the preferred agent in patients with hyperthyroidism because it has a longer intrathyroidal half-life than PTU and usually can be administered once daily.

Radioactive iodine is contraindicated during pregnancy because it can cause destruction of the fetal thyroid.

Thyroidectomy is generally avoided during pregnancy unless the hyperthyroidism cannot be controlled or the patient cannot tolerate either PTU or methimazole. When needed, surgery is typically performed during the second trimester, if possible.