It may be difficult to confirm a suspected diagnosis of Graves’ disease during an otherwise normal pregnancy. Much of the difficulty that arises may be related to changes in the structure and function of the maternal thyroid gland that occur as a normal adaptation to pregnancy. During the first trimester of pregnancy, the maternal thyroid gland usually enlarges to a certain extent. In some women, it may enlarge to the point where increased blood flow may be heard as a rush when a stethoscope is held up against the thyroid gland. As the placenta grows and develops during pregnancy, it begins to produce and secrete large amounts of the hormone estrogen. Increased amounts of estrogen stimulate the liver to produce increased amounts of a protein called thyroid hormone-binding globulin that binds to thyroid hormone as it travels through the bloodstream. As a result of this, the total amount of thyroid hormone in the bloodstream usually increases. The amount of free thyroid hormone present in the bloodstream usually stays the same. The placenta also produces and secretes large amounts of a hormone called human chorionic gonadotropin, also known as hCG. This hormone is somewhat unique in that it able to bind to the TSH receptors present on the surface of follicular cells of the thyroid gland. By doing so, hCG may mimic the action of TSH, stimulating follicular cells to produce and secrete increased amounts of thyroid hormone. In some cases, binding of hCG to TSH receptors may actually induce a mild form of hyperthyroidism that leads to mild suppression of TSH levels.
As a result of all of these changes, a woman who is progressing through a perfectly normal pregnancy may present with findings that appear to be consistent with Graves’ disease, including an accelerated heart rate, enlarged thyroid gland, increased total thyroid hormone levels, and suppressed TSH levels. Distinctions may be even more difficult in pregnant women who present with severe nausea, vomiting, weight loss, and dehydration. Questions may arise as to whether the symptoms and signs reflect the effects of exposure to excess levels of thyroid hormone due to underlying hyperthyroidism, or whether the laboratory test results reflect the effects of exposure to excessive levels of hCG.
Unfortunately, thyroid uptake studies cannot be used to provide an estimate of the thyroid gland's level of activity during pregnancy. Exposure to radioactive iodine could cause irreversible damage to the fetal thyroid gland. In light of this, accurate diagnosis of Graves’ disease during pregnancy usually relies on a careful assessment performed by an experienced doctor. In taking a history, the doctor may focus on question that seek to determine whether symptoms and signs concerning for thyrotoxicosis may have been present for any length of time before the patient became pregnant. Other questions may focus on whether the patient has experienced any symptoms consistent with possible thyroid eye disease. Physical examination may focus on determining whether changes in the size and character of the patient's thyroid gland are greater than might be expected as a result of normal adaptation to pregnancy. Particular attention may be paid to searching for signs of thyroid eye disease, pretibial myxedema, or changes in the texture of the patient's skin. Review of laboratory tests may focus on analysis of different patterns of changes to see whether any disturbances may be consistent with abnormal production and secretion of excess amounts of thyroid hormone. Specific attention may be paid to changes in levels of free thyroid hormone. In some cases, it may be necessary to follow certain laboratory tests over time to see if certain patterns of changes persist to confirm or rule out a suspected diagnosis.