Morning sickness and thyroid function in normal pregnancy.

Thyroid function in early normal pregnancy was evaluated with reference to morning sickness using a newly developed free thyroxine (T4) radioimmunoassay and a highly sensitive TSH immunoradiometric assay. A significant increase in serum free T4 and a decrease in serum TSH were observed in early pregnancy relative to the levels in nonpregnant controls. The increased free T4 and hCG and decreased TSH correlated with the severity of morning sickness, and these changes were especially marked in subjects with nausea and vomiting. The individual serum levels of hCG in the pregnant group correlated significantly, directly with the levels of free T4 and inversely with those of TSH. The increased free T4 and decreased TSH in subjects with emesis returned to the normal ranges of nonpregnant controls after improvement of emesis. These data indicate that the thyroid gland is physiologically activated in early pregnancy, possibly by hCG or a related substance, which may induce gestational emesis. On the other hand, an increased level of free T4 and a reduced level of TSH in early normal pregnancy are not indications of thyrotoxicosis and may not necessitate antithyroid drug treatment.