Expression of the thyroid hormone receptor (TR beta), the 90 KD heat shock protein (HSP 90), and the oncogenes H-ras and c-myc mRNA in normal hyperplastic, and neoplastic human thyroid tissue was investigated by Northern blot and slot blot analyses. The TR beta mRNA was present in all normal and neoplastic thyroid tissue samples. The levels were significantly higher in normal and hyperplastic tissues (7.91 +/- 0.48 and 7.60 +/- 0.68 arbitrary units, respectively) than in neoplastic tissues (3.82 +/- 0.67) (p < 0.001). H-ras and c-myc mRNA were also detected in glandular tissue specimens, but no significant difference was observed in their expression levels. Furthermore, there was a tendency to a negative correlation between the level of TR beta and c-myc mRNA (p = 0.06). In normal thyroid tissue, HSP 90 mRNA levels were significantly higher than in hyperplastic and papillary carcinoma tissue specimens (p < 0.001). These results indicate that mRNA of TR beta and HSP 90 (the latter only in papillary thyroid carcinoma) are expressed in relation to the degree of cellular differentiation. Furthermore, the presence of TR beta in normal thyroid tissue implies that T3 and T4 may be involved in the regulation of their own production via TR beta activated feedback mechanism(s).