Recent controversial studies in mice imply that TSH inhibits bone remodelling, suggesting TSH-deficiency rather than thyroid hormone excess causes bone loss in thyrotoxicosis. Measurement of the TRH-stimulated rise in TSH following three days administration of graded doses of T3 is the gold-standard for determining resistance to thyroid hormone (RTH). During this test, T3 excess coupled with suppressed TSH should induce high bone turnover, whilst administration of TRH causes a rapid rise in circulating TSH. Thus, we hypothesised that if TSH inhibits bone remodelling this would result in suppression of bone turnover. Following ethical approval, TSH, osteocalcin and type I collagen C-terminal telopeptide (CTX) responses to TRH were determined during the graded T3-suppression test in eight volunteers (6 male, 2 female, 2644 years) and two subjects with RTH: (1) female aged 60, T3 receptor β mutation TRβF455L, (2) male, 54, TRβG332Q. Following TRH-stimulation, mean peak TSH (mU/L) in volunteers was 8.39±3.13 at baseline, 1.21±0.40 following 50 μg T3, 0.23±0.16 following 100 μg T3 and 0.06±0.06 following 200 μg T3. In subjects 1 and 2, TSH levels rose to 8.43 and 104.8 (baseline), 5.41 and 77.21 (50 μg T3) and 2.83 and 35.23 (100 μg T3), respectively, and to 13.18 mU/L following 200 μg T3 in subject 2. In volunteers, osteocalcin (ng/mL) increased significantly (P<0.01) following each dose of T3; 19.9±1.8 (baseline), 23.9±1.8 (50 μg T3), 27.3±2.1 (100 μg T3) and 29.7±1.4 (200 μg T3). In contrast, baseline osteocalcin levels were lower (P<0.01) in RTH subjects [(1) 16.3±4.5; (2) 11.8±1.4] and were unaffected by T3. TRH-stimulated rises in TSH did not alter osteocalcin in volunteers or RTH subjects up to 48 hours following TRH. Similarly, normal diurnal variations in CTX levels were unaffected by TRH-stimulated rises in TSH in both volunteers and RTH subjects. These data indicate that T3 rather than TSH is the physiological regulator of bone turnover in man.