Low in vitro thyrotropic activity of a human chorionic gonadotropin molecule in the first trimester during pregnancy

Rat thyroid cell strain FRTL-5 is widely used to analyze thyroid stimulators, and responds to purified hCG with increased cyclic AMP production. The aim of this study was to determine the in vitro thyrotropic activity of an immunological hCG molecule according to gestational age during normal pregnancy using the FRTL-5 cell culture system. Sera were collected from 7 normal nonpregnant control and a total of 72 normal pregnant women. Seventy-two pregnant women consisted of 40 women in the first trimester (7-14 weeks' gestational age (GW) (group I)), 16 women in the second trimester (15-28 GW, group II), and 16 women in the third trimester (29-40 GW, group III). Concentrations of serum hCG and FT4 of group I were higher than those of groups II and III (p < 0.001). In the in vitro assay, the supernatants were subjected to radioimmunoassay for cAMP. In order to analyze in vitro thyrotropic activity of an immunological hCG molecule, cAMP accumulation rate expressed as cAMP accumulation (nM) divided by maternal serum concentration of hCG (IU/ml) was determined in all three groups. By regression analyses, a statistically significant positive correlation was observed between serum hCG concentration and cAMP accumulation in group I (r = 0.399; p < 0.05), group II (r = 0.915; p < 0.0001) and group III (r = 0.671; p < 0.01). cAMP accumulation rate in group I (0.355 +/- 0.022, mean +/- SE) was significantly low when compared with group II (0.508 +/- 0.051) (p < 0.01) and group III (0.556 +/- 0.063) (p < 0.001). Thus, it was found for the first time that the in vitro thyrotropic activity of an immunological hCG molecule in the first trimester was lower than those of the second and the third trimesters. In terms of the in vitro thyrotropic activity, an immunological hCG molecule in the first trimester differs from that of the later trimesters.