Hyperthyroidism is a condition characterized by the overproduction of thyroid hormones, triiodothyronine (T3) and thyroxine (T4), resulting in an overstimulated metabolic state. During pregnancy, hyperthyroidism is relatively rare, affecting approximately 0.1% to 0.4% of pregnancies. The most common cause of hyperthyroidism in pregnancy is Graves’ disease, accounting for 85% to 95% of clinically significant cases. Graves’ disease is an autoimmune disorder where thyrotropin receptor antibodies (TRAb) stimulate the thyroid gland by binding to thyroid-stimulating hormone (TSH) receptors, leading to excessive thyroid hormone production. This overstimulation causes hyperthyroidism and systemic symptoms that affect multiple organ systems.
Other less common causes of hyperthyroidism in pregnancy include toxic multinodular goiter, autonomous thyroid nodules, gestational transient thyrotoxicosis (GTT), and thyroiditis. Gestational transient thyrotoxicosis occurs due to hormonal changes in early pregnancy, particularly elevated levels of human chorionic gonadotropin (hCG), which can stimulate the thyroid because of its structural similarity to TSH. GTT affects about 1% to 3% of pregnancies (more with hyperemesis gravidarum and multiple gestation), which makes it the most common cause of a suppressed TSH in early pregnancy, more common than Graves' disease itself. Unlike Graves' disease it has no goiter, eye disease, or TRAb; it resolves on its own by about 14 to 18 weeks as hCG falls, and it is managed with supportive care and, if needed, a short course of a beta-blocker rather than antithyroid drugs. While rare in women under 40, toxic adenomas or thyroid nodules that autonomously produce thyroid hormones may also contribute to hyperthyroidism in pregnant women.
