The thyroid is a butterfly-shaped gland at the base of the throat, below the voice box (larynx) and above the sternum (breastbone). It produces several thyroid hormones, most importantly T3 (triiodothyronine) and T4 (thyroxine). Thyroid hormones regulate important metabolic functions in the body, including thermal balance and heart rate, as well as the rate of breakdown of sugars and fats in the liver and other organs of the digestive tract.

During normal pregnancy, the body produces increased levels of pregnancy hormones which - amongst other things - activate thyroid hormone secretion and enlarge the size of the thyroid by as much as 10% to 30%. Specifically, human chorionic gonadotropin (hCG) from he placenta stimulates the thyroid to secrete T3 and T4 which in turn down regulate secretion of thyroid stimulating hormone (TSH) in the anterior pituitary gland. At the same time, higher levels of estrogen released from the ovaries stimulate production of thyroid hormone binding protein, which binds free T3 and T4 in the body. As a result, even though levels of T3 and T4 are elevated, the number of actively operating hormones does not increase during normal pregnancy.

However, increased levels of T3 and T4 can lead to pathologies in women with preexisting (though often symptom-free) thyroid disorders. In such cases, increased levels of thyroid hormones trigger previously dormant thyroid disorders to flare up during pregnancy. This is because such disorders are auto-immune diseases that cause abnormal production of antibodies by the immune system (thyroid peroxidase, TPO, causing hypothyroidism, and thyrotropin receptor antibody, TRAb, causing hyperthyroidism). These antibodies can up or down regulate thyroid function. Because hormone levels are elevated during pregnancy, antibody levels rise as well, eventually causing pathologic symptoms. It is estimated that 2% to 3% of pregnancies are affected thyroid disfunction (with hypothyroidism being much more common than hyperthyroidism).

The two most common auto-immune diseases underlying thyroid problems during pregnancy are Graves' disease (causing hyperthyroidism, i.e. overproduction of thyroid hormones) and Hashimoto's disease (causing hypothyroidism, i.e. underproduction of thyroid hormones). Because of the opposing nature of these disorders, patients with hyper- and hypothyroidism usually exhibit opposing sets of symptoms. Severe cases of both diseases can be dangerous for mother and fetus. However, there are effective medical treatments for both conditions. Moreover, if managed correctly, most patients can eventually discontinue medication at some point after pregnancy. It must be noted though that the underlying condition is chronic and will likely flare up again with any subsequent pregnancy.