Administration of recombinant human tumor necrosis factor-α (TNF) to rats and mice produces a model of nonthyroid illness in which there is impairment of hypothalamic- pituitary thyroid function, including reduced serum concentrations of T₄ and T₃, reduced thyroid radioiodine uptake, and reduced response to TSH. In this study, we tested the binding and effects of TNF on FRTL-5 cells and on four human thyroid carcinoma cell lines. The TSH-stimulated [¹²⁵I]iodide uptake by FRTL-5 cells was inhibited by TNF in a dosedependent manner. The four human thyroid carcinoma cell lines (NPA, MRO, ARO, WRO) have TSH receptors but did not respond to TSH in regard to iodide uptake and thymidine incorporation. Both human thyroid carcinoma cells and FRTL-5 cells contain specific receptors for TNF. Scatchard analysis showed that the receptor numbers and dissociation constants in human thyroid carcinoma cells and FRTL-5 cells were, respectively; 2.4 × 10⁴, 5.4 nM (WRO); 8 × 10³, 3.4 nM (MRO); 4 ×lO³, 1 nM (ARO); 7 × 10³, 1 nM (NPA); 3 × 10³, 1 nM (FRTL-5), and 9 × 10³,1 nM (FRTL-5 cells treated with TSH). The results indicate that TNF affects thyroid cell function through binding to the TNF receptor and that the number of TNF receptors is regulated by TSH.