The genes for either the TSH receptor (TSH-R) or the stimulatory guanine nucleotide-binding protein subunit (Gs alpha) can undergo somatic mutations in thyroid cells, leading to constitutive activation of adenylyl cyclase and the formation of clonal hyperfunctioning thyroid adenomas. Autonomously functioning thyroid adenomas are thought not to be common precursors of thyroid cancer. If this is the case, mutations of the TSH-R or Gs alpha would not be expected to be highly prevalent in thyroid carcinomas. In this paper we report the results of a screen for structural defects in exon 10 of the TSH-R (which includes the whole serpentine structure, but not the extracellular domain) and of Gs alpha in 30 thyroid carcinomas. Five of these were from patients with functioning metastasis, as we hypothesized that if mutations of these genes were to play a role in the progression to malignancy, they would be more likely to manifest in thyroid cancers that retain unusual differentiated function (i.e. capable of synthesizing enough thyroid hormone to render patients euthyroid or hyperthyroid after total thyroidectomy). None of the 30 tumors had activating point mutations of Gs alpha. Only 2 of 30 had somatic mutations of the TSH-R (codon 632: ACC to GCC, Thr to Ala; and ACC to ATC, Thr to Ile, respectively), the latter in a patient with a thyroid hormone-producting follicular carcinoma. These results suggest that events leading to constitutive activation of the adenylate cyclase signal transduction cascade are not a frequent event in the progression toward differentiated thyroid carcinomas.