Increased hypothalamicpituitarythyroid (HPT) axis activity results in high bone turnover. T3 stimulates osteoblast and osteoclast activities, whereas TSH is proposed to inhibit bone turnover directly. Resolving the relative importance of T3 and TSH is complicated by their physiological inverse relationship. We studied 10 controls and 4 patients with resistance to thyroid hormone (RTH), in which mutation of thyroid hormone receptor beta (THRB) disrupts HPT-axis negative feedback. RTH is characterized by increased T4, T3 and TSH concentrations and a TSH response to TRH-stimulation that is not suppressed by T3. We hypothesized that skeletal effects of T3 and TSH could be discriminated by comparing bone formation (osteocalcin) and resorption (C-telopeptide of type I collagen (CTX)) responses to graded T3-suppression-TRH-stimulation testing in control and RTH subjects. Fasting samples were collected prior to TRH administration and for 6 h. Testing was repeated three times following increasing doses of T3 (50, 100, 200 μg/day for 3 days), and TSH, osteocalcin and CTX were determined in all samples. Baseline TSH increased 5.81±0.29 fold (P<0.0001) in controls and 4.06.8 fold (P<0.0001) in RTH patients following TRH-stimulation. TSH responses to TRH were suppressed by T3 in control subjects but not RTH patients. Baseline osteocalcin (P<0.0001) and CTX (P<0.0001) were higher in controls and T3-treatment increased osteocalcin 1.55±0.08 fold (P<0.0001) and CTX 1.37±0.21 fold (P<0.05). In RTH patients, osteocalcin and CTX responses to T3 were impaired and variable, reflecting heterogeneity of the condition and the different THRB mutations identified. By contrast, osteocalcin and CTX levels in all subjects remained unchanged following TRH administration. These data demonstrate that T3 stimulates bone turnover, whereas the rise in TSH following TRH-stimulation does not inhibit osteocalcin or CTX. Thus, rapid bone turnover responses to increased HPT-axis activity result from stimulatory actions of thyroid hormones and not the loss of any inhibitory effects of TSH.