Biochemical markers of bone remodelling were used to evaluate bone turnover in two types of autosomal dominant osteopetrosis (ADO) at baseline and during stimulation with triiodothyronine (T3). Eight patients with Type I (aged 23-61 years, mean 40.4 years) and nine patients with Type II ADO (aged 20-49 years, mean 32.8 years) were compared with 10 normal controls (aged 22-54 years, mean 35.4 years). The participants were treated with 100 microg T3 daily for 7 days and followed for a total of 16 weeks. Serum concentrations of T3 increased and corresponding suppression of TSH was observed in all participants. Both formative and resorptive bone markers were normal at baseline. After stimulation with T3, a significant increase was seen in all groups for the formative markers used. Secondary increments were observed at the end of the observation period for all groups, indicating activation of bone remodelling. A variety of resorptive markers was assessed, but no differences between patients and controls were seen. After stimulation, highly significant responses were found in all parameters in all groups, in accordance with stimulation of existing resorptive cells. However, no secondary increments were seen at the end of the observation period. A more pronounced response was found in crosslinks-related assays. The study demonstrates that it is possible to stimulate bone resorptive and formative cells with thyroid hormones in both types of ADO. Moreover, it indicates that the remodelling process is activated by a short course of T3 treatment.