Gestational Hyperthyroidism/Transient HyperthyroidismThe most common cause of hyperthyroidism in pregnancy, however, is gestational hyperthyroidism, also called “transient hyperthyroidism.” This type of hyperthyroidism is characterized by elevated Free T4, and low or undetectable thyroid stimulating hormone (TSH) level, without any antibodies that would suggest Graves’ disease. It’s estimated that this type of hyperthyroidism occurs in 1% to 3 % of pregnancies, and is due to elevated hCG, a pregnancy hormone.
In some women, gestational hyperthyroidism can be a cause for a condition known as hyperemesis gravidarum, which is characterized by severe nausea, excessive vomiting, electrolyte disturbances, and weight loss of more than 5% of body weight. The condition usually occurs during the first trimester of pregnancy, and after hCG levels peak during the first trimester, thyroid levels typically normalize.
According to the 2011 "Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum,” due to the effect of hCG, normal TSH values during pregnancy can be as low as 0.03 mIU/L. The Guidelines recommend that in a woman who has a TSH level of less than .1 mIU/L during her first trimester, doctors should take a medical history and perform a physical exam and and TSH and Free T4 measured.
Typically, in the absence of clinical signs of Graves’ disease -- like a goiter, or thyroid eye-symptoms/ophthalmopathy – doctors will diagnose gestational hyperthyroidism. But if there is a suspicion, the Guidelines recommend measuring TSH receptor antibody (TRAb) levels.
According to the Guidelines, radioactive iodine (RAI) scanning or radioiodine uptake (RAIU) tests should not be done in pregnancy, as they are a risk to fetal thyroid development.
Antithyroid drugs are not recommended for gestational hyperthyroidism. For women who have gestational hyperthyroidism along with hyperemesis gravidarum, the treatment recommendations include management of dehydration, and hospitalization if needed.
Stagnaro-Green, Alex, et. al. "Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum.” Thyroid. Volume 21, Number 10, 2011 (Online)