[Study on serum TSH and its response to TRH measured by a high sensitive immunoradiometric assay during and after...

With a new highly sensitive immunoradiometric assay (IRMA), serum thyrotropin (TSH) concentrations were determined in 213 healthy pregnant women. Serum free thyroxine (FT4) and human chorionic gonadotropin (HCG) levels were also measured in the same individuals. The mean serum TSH value in the 1st trimester was 0.8 microU/ml and significantly lower than those of the other three periods and non-pregnant women. Seventeen of 77 (22.1%) and 2 of 128 (1.6%) subjects in the 1st and 3rd trimester, respectively, had an undetectable serum TSH value (less than 0.1 microU/ml) with normal or slightly elevated serum FT4 levels and these pregnant women had no clinical signs or symptoms of thyrotoxicosis. Significant positive correlations were found between serum FT4 and HCG in the 1st and 3rd trimester. In 13 subjects whose serum basal TSH values had been less than 0.1 microU/ml, the thyrotropin-releasing hormone (TRH) stimulation test was performed. Two of 4 subjects in the 1st trimester and one of 8 subjects in the 2nd trimester did not respond to TRH and their serum TSH values before TRH administration were less than 0.1 microU/ml. Although the exact mechanism of blunted TSH response to TRH is not clear, the feedback suppression of serum TSH by slightly elevated serum FT4 concentrations may occur early in pregnancy. However, in the 3rd trimester serum FT4 values fell below the reference range without an elevation of serum TSH. Other factors regulating the secretion of TSH during pregnancy can be postulated.