Spontaneously transformed somatic thyrocyte mutants, FRTL-5/TA and FRTL-5/TP, are thyrotropin (TSH) independent for growth and show loss of the thyroid-specific phenotype, with absent thyroglobulin and thyroid peroxidase gene expression. To investigate the role of TSH-receptor (TSH-R) activation in rat thyroid growth and function, binding of TSH and TSH-induced cAMP production were measured in intact cells under identical assay conditions. TSH binding did not differ in terms of affinity and receptor number and presence of 5.6 kb and 3.3 kb mRNA rat TSH-R transcripts was determined in all variants. By contrast, basal cAMP was 11-fold lower in FRTL-5/TA and 6-fold lower in FRTL-5/TP than in wild-type FRTL-5 (1.1 +/- 0.4; P < 0.01). Maximal cAMP production was similar between wild-type and cell variants and stimulation by bovine, rat and recombinant human TSH revealed normal activation patterns. Therefore, a dissociation was present between the loss of TSH control on growth and function, and the presence of a normally functioning TSH-R. Subsequent to TSH incubation FRTL-5/TP and FRTL-5/TA cells showed a different expression pattern of TSH-R and the proto oncogenes c-myc and fos than FRTL-5 wild-type. The data indicated that the cause of the TSH-independency is located down-stream of the cAMP cascade, influencing genes that control the expression of cell cycle-related proto-oncogenes and thyroid-specific genes.