The percentage of cells in the S/G2M fraction and the cytosol deoxythymidine kinase activity (TKA) were measured in autonomously functioning thyroid adenomas (AFTA) and paranodular tissue (PNT), with special regard to the impact of the patient's serum TSH concentration on DNA synthesis. The S/G2M fraction was determined by means of DNA flow cytometry, and TKA was determined by radioenzyme assay. The S/G2M fraction of AFTA (n = 15, median; 7.1%; range, 3.2-9.2%) exceeded the S/G2M fraction of normal thyroid tissue (n = 8; median, 2.8%; range, 2.3-4.0%; P = 0.008) and in 12 of 13 AFTA was 1.2- to 2.3-fold higher than the S/G2M fraction in the corresponding PNT (median, 4.0%; range, 2.5-6.7%; P = 0.0022). TKA of AFTA (n = 15; median, 681 microIU/mg; range, 432-854 microIU/mg) exceeded TKA of normal thyroid tissue (n = 8; median, 356 microIU/mg; range, 194-426 microIU/mg; P = 0.0001) and was 1.1- to 4.2-fold increased compared with TKA activity in the corresponding PNT (median, 430 microIU/mg; range, 162-570 microIU/mg; P = 0.001). In the absence of measurable serum TSH there was a constant increase in the S/G2M fractions and the TKA in AFTA vs. those in PNT. In patients treated with methimazole with serum TSH concentrations of 0.5 mIU/L or more, the S/G2M fraction and TKA in both AFTA and PNT were significantly higher than those in untreated patients with serum TSH concentrations of 0.5 mIU/L or less. In the majority of AFTA, functional autonomy and increased DNA synthesis are concomitant phenomena. Although TSH may stimulate DNA synthesis in both AFTA and PNT, measurable serum TSH concentrations are apparently not essential for DNA synthesis.