We reported that gestational thyrotoxicosis is induced by thyroid-stimulating activity (TSA) of circulating hCG. However, the serum immunological hCG concentration did not correlate to TSA. To elucidate this, we examined the relation of carbohydrate moieties of hCG to bioactivity in 79 early pregnant women, divided into 4 groups: no emesis, mild emesis, hyperemesis, and gestational thyrotoxicosis with hyperemesis. Serum free T4 (FT4) and free T3 (FT3) levels were significantly higher and TSH was lower in the hyperemesis (FT4, 23.42 +/- 5.02 pmol/L; FT3, 6.26 +/- 1.80 pmol/L; TSH, 0.30 +/- 0.44 mU/L) and in gestational thyrotoxicosis (FT4, 48.65 +/- 14.80 pmol/L; FT3, 14.71 +/- 3.47 pmol/L; TSH, < 0.04 mU/L) groups than in the no emesis group (FT4, 16.99 +/- 2.48 pmol/L; FT3, 5.51 +/- 0.75 pmol/L; TSH, 1.37 +/- 1.23 mU/L; P < 0.0005). TSA was also significantly higher in the hyperemesis (566 +/- 187%) and gestational thyrotoxicosis (832 +/- 168%) groups than in the no emesis group (321 +/- 135%). We found no significant difference among serum hCG concentrations measured by immunoassay in the four groups. To characterize the carbohydrate chains, serum hCG was fractionated by Concanavalin-A and ricin lectin affinity chromatography. The fraction firmly bound to Con-canavalin-A, which contains hCG with high mannose and hybrid-type carbohydrate chains, was significantly higher in the hyperemesis group (91.07 +/- 2.06%; n = 15) than in the no emesis group (89.61 +/- 2.38%; n = 24; P < 0.04). The fraction firmly bound to ricin column, which contains hCG with asialo-carbohydrate chains, was significantly increased in the gestational thyrotoxicosis group (3.44 +/- 1.70%; n = 5) compared with that in the no emesis group (1.77 +/- 0.49%; n = 24; P < 0.03). Serum FT4 positively correlated to the hCG fraction firmly bound to ricin column (r = 0.61; P < 0.001). We conclude that thyrotoxicosis with hyperemesis may be caused by circulating asialo-hCG with higher thyrotropic bioactivity.