Triiodothyronine (T3) nuclear binding sites in human placenta and decidua

We studied the binding of triiodothyronine (T3) to human placenta and decidua. Although the placenta effectively blocks transfer of thyroxine (T4) and T3 it may itself be a thyroid hormone-dependent tissue and may specifically bind T3. A single class of high-affinity binding sites was found (Ka 3.8 +/- 0.31 s.e.m., and 3.3 +/- 10(9) M-1) for both placenta and decidua. Limited capacity was also observed, viz., 0.117 +/- 0.01 and 0.102 +/- 0.018 fmoles/microgram deoxyribonucleic acid (DNA), respectively. Endogenous T3 nuclear saturation was 34 per cent. Our results are consistent with the proposal that placenta and decidua have similar T3 specific nuclear binding sites and that T3 may have a direct action in the placenta.