Human Chorionic Thyrotropin: Further Characterization and Study of Its Secretion During Pregnancy1

Abstract

The partial purification and immunological characterization of the human chorionic thyrotropin (HCT) was described. The behavior of the chorionic factor during the steps of purification was similar to that of pituitary thyrotropin (TSH) although its chromatographic and electrophoretic properties suggested that HCT was less acidic than the human pituitary hormone. The specific activity of the most purified fraction was 3500 times that of the starting extract. Double diffusion experiments showed the extent of immunological cross-reactivity of HCT with human pituitary thyrotropin to be low, in confirmation of earlier results, but a close immunological relationship between the chorionic factor, bovine and porcine TSH was found when both were reacted against highly specific antibovine or antiporcine TSH serum. This rather surprising difference in the degree of cross-reactivity between HCT with human compared to that with porcine and bovine TSH was confirmed by experiments on the neutralization of bio logical activity and by radioimmunological studies: HCT interfered very poorly in a 131I-human TSH-antihuman TSH antibody system, while it strongly inhibited the binding of labeled bovine TSH to antibovine or to antiporcine TSH antibody; HCT also inhibited the binding of labeled human TSH to antiporcine TSH antibody. The circulating thyroid-stimulating activity in pregnant women was found significantly higher than in serum of nonpregnant women, and particularly at the beginning of gestation. Radioimmunoassay of pregnancy serum extracts with high thyroid-stimulating activity gave results fitting in with the behavior of HCT in the 131I-human TSH-antihuman TSH as well as in the 13lI-human TSH-antiporcine TSH systems. It was thus suggested that HCT is actually secreted during pregnancy and responsible for the high thyroid stimulating activity of pregnancy serum.