In normal early pregnancy, serum free thyroxine (T4) increases and serum TSH decreases, indicating that the thyroid gland is activated physiologically. To identify the factor responsible for this thyroid activation, we measured the serum thyroid-stimulating activity in comparison with the serum level of hCG in 39 normal women in early pregnancy. Serum thyroid-stimulating activity was measured by a sensitive cyclic adenosine 3'5'-monophosphate (cAMP) accumulation assay using a rat thyroid cell line (FRTL-5). Thyroid- stimulating activity was detected in 37 women (95%), and the activities of individuals correlated positively with their serum free T4 levels (r=0.474; P<.01) and negatively with their serum TSH levels (r=-0.376; P<.02). Moreover, serum thyroid-stimulating activity correlated closely with the serum hCG level (r=0.741; P<.001), but was completely abolished by pre-treatment of the sera with hCG antibodies. These data indicate that in normal early pregnancy, the thyroid gland is physiologically activated by serum hCG, which has intrinsic thyroid-stimulating activity.
(C) 1990 The American College of Obstetricians and Gynecologists