We studied the effects of long-term suppressive thyroxine treatment on serum markers of bone collagen synthesis and degradation in thyroid carcinoma patients, and the relationship of these effects to the serum concentrations of thyrotropin, free thyroxine and free triiodothyronine were investigated. Thirty-seven thyroid carcinoma patients receiving a stable thyroxine dose, and thirty-five controls participated in a cross-sectional study. Bone collagen synthesis and degradation were measured by using specific radioimmunoassays to determine the serum concentrations of carboxyterminal propeptide of type I procollagen, and carboxyterminal telopeptide region of type I collagen, respectively. Serum thyrotropin, free T4 and free T3 were measured by time-resolved fluoroimmunoassays (Delfia). Serum carboxyterminal telopeptide region of type I collagen concentrations of thyroid carcinoma patients were significantly higher than those of the controls (p = 0.0012). Serum carboxyterminal propeptide of type I procollagen concentrations did not differ significantly between the patients and controls (p = 0.85). Significant associations between age or physical activity, and carboxyterminal propeptide of type I procollagen or carboxyterminal telopeptide region of type I collagen were found in the controls, but not in the patients. Thyrotropin, free T4 or free T3 were not significantly associated with carboxyterminal propeptide of type I procollagen or carboxyterminal telopeptide region of type I collagen in either the control or patient group. From these results it is concluded that long-term suppressive thyroxine treatment seems to accelerate bone degradation, but not bone formation, and therefore carries a risk for osteoporosis.(ABSTRACT TRUNCATED AT 250 WORDS)