Stimulation of cyclic AMP production in FRTL-5 thyroid cells by crude immunoglobulin fractions of serum from pregnant women.

In an assay for thyroid-stimulating antibodies, in which FRTL-5 thyroid cells were incubated with crude immunoglobulin (Ig) fractions precipitated from serum with 15% polyethylene glycol, significant increase in cAMP production was elicited by the samples from 25 (35.7%) out of 70 pregnant women. The highest value was 529.5%. There was a close correlation between thyroid stimulating activities and serum hCG concentrations (r = 0.708, P less than 0.001). When 125I-hCG was added to serum from pregnant women, about 20% of the radioactivity was incorporated into the Ig fractions. hCG preparations within a range of concentrations of 30-300 IU/m elicited 2.3-16.5 times increase in cAMP in a dose-dependent manner. Nine pregnant women with serum TSH concentrations less than the lower limit of the normal range (less than 0.25 mU/l) displayed significantly higher values for both thyroid stimulating activities and serum hCG concentrations (P less than 0.001, respectively) compared with those who had normal TSH levels in serum. These data suggest that hCG or its variant may stimulate the thyroid sufficiently to suppress secretion of TSH from the pituitary in some pregnant women.