Nuclear thyroxine and 3,5,3'-triiodothyronine receptors in human mononuclear blood cells during pregnancy.

Serum free T4, free T3, TSH and maximal nuclear binding capacity for T4 and T3 in mononuclear blood cells were measured in 12 control women, in 12 normal pregnant women in the first and in 12 women in the third trimester. Serum free T4 and T3 were decreased in late pregnancy compared to control women, serum free T4: 9.1 pmol/l(mean) vs 12.9 pmol/l(mean); serum free T3: 4.0 pmol/l(mean) vs 6.2 pmol/l(mean), without any change in TSH levels: 2.2 mU/l(mean) vs 1.8 mU/l (mean). Concomitantly, the maximal nuclear binding capacity for both T4 and T3 increased, T4: 2.7 fmol T4/100 micrograms DNA vs 1.8 fmol T4/100 micrograms DNA; T3:2.8 fmol T3/100 micrograms DNA vs 2.0 fmol T3/100 micrograms DNA. These data, obtained from healthy women during a normal pregnancy are compatible with mild compensated hypothyroidism. We suggest that euthyroidism are maintained by the increased maximal nuclear binding capacity for these hormones.