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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Thyroid Disorders in Pregnancy
Sponsored by The Indian Health Service Clinical Support Center
2. Background
Thyroid disorders are common
in young women and may first come to attention when the patient presents for pregnancy care. Pregnancy symptoms may mimic the clues to thyroid disease and prompt laboratory valuation. Various physiologic changes in pregnancy make evaluation of thyroid function during gestation different from that in nongravid women.
Thyroid binding globulin (TBG) is increased in pregnancy as a result of estrogen induced stimulation of its synthesis. This results in seemingly abnormally high values for total thyroxine (total T4) and total triiodothyronine (total T3), and low values for the resin T3 uptake (RT3U), commonly ordered tests for the evaluation of thyroid function in non-pregnant women.
Thyroid stimulating hormone (TSH) values must be also interpreted with caution during pregnancy. The newer third generation monoclonal immunoassays for TSH may be spuriously lowered during pregnancy. The increase in chorionic gonadotrophin (hCG) during pregnancy may inhibit hypothalamic thyrotropin releasing hormone (TRH) and result in a reduction of maternal TSH levels, sometimes to undetectable levels, making it appear that the patient has hyperthyroidism, even though she remains clinically euthyroid. There is close structural homology between both the hCG and the TSH molecules and their receptors, and this may explain these effects.
To make it more interesting, the thyroid glands of normal pregnant women may be stimulated by high levels of hCG to secrete slightly excess amounts of T4 and to undergo a further suppression of TSH (see below for more on this). The stimulation of the high levels of hCG may also result in a physiologic increase in size of the gland. Free thyroxine (free T4) levels remain the most reliable test of thyroid function during pregnancy.
