Serum thyrotropin by ultrasensitive immunoradiometric assay and serum free thyroid hormones in pregnancy.

Variations of serum TSH, measured by an ultrasensitive immunoradiometric assay, of serum total and free thyroid hormones and of thyroxine-binding globulin (TBG) and sex hormone-binding globulin (SHBG) were investigated in a group of 18 normal women before and during pregnancy. A gradual increase of total thyroid hormones, TBG and SHBG was observed, while mean serum free thyroxine and free triiodothyronine progressively decreased. Serum TSH concentrations were comprised within the normal range throughout pregnancy, although a small but significant increase was found in the 2nd and 3rd trimester. These changes may represent a compensatory mechanism to meet the increased demand for thyroid hormones in pregnancy and must be taken into account for a correct evaluation of thyroid function during gestation.