Human thyrotropin (TSH) receptor steady-state transcript levels were analyzed by Northern blot analysis in thyroids of patients with thyroid carcinoma, with hyperfunctioning adenoma and in normal controls. In control tissue and benign tumors expression levels of TSH receptor mRNA were high whereas in anaplastic carcinomas no normal TSH receptor mRNA was detected. In papillary and follicular tumors it varied from normal to markedly reduced levels. Thyroid peroxidase (TPO) and thyroglobulin (Tg) mRNA were strongly expressed in normal tissue and in hyperfunctioning adenomas but were completely lost in all anaplastic tumors. In papillary tumors expression of TPO and Tg mRNA varied from normal to a complete loss of expression of either TPO, Tg or both. Tg and TPO steady-state expression did not correlate to TSH receptor transcript levels. C-myc mRNA was highly expressed in anaplastic carcinomas, very variable in normal controls and in differentiated thyroid tumors and low in hyperfunctioning adenomas. In summary, TSH receptor mRNA is persistently expressed in all differentiated thyroid tissues and tumors but lost in undifferentiated carcinomas. Its persistence far along the transformation pathway further supports the concept that this gene which inserts the thyrocytes in the physiological regulatory network is almost constitutively expressed in this cell.