Langdahl BL, Loft AGR, Eriksen EF, Mosekilde L, Charles P. Bone mass, bone turnover and body composition in former hypothyroid patients receiving replacement therapy. Eur J Endocrinol 1996;134:702–9. ISSN 0804–4643 The aim of the present cross-sectional study was to disclose whether long-term thyroxine replacement therapy (TRT) in primary hypothyroidism causes osteopenia. We compared 36 adult biochemically and clinically euthyroid patients who had received TRT for more than 5 years (mean 13 years) for primary hypothyroidism with 80 sex- and age-matched normal controls. Height, body weight and lean body mass were similar, but the patients had 21% higher fat body mass (p < 0.01) than their controls. Furthermore, compared to controls the patients had 29% higher serum thyroxine (T₄) and 31% higher serum free T₄ index (FT₄I) levels (p < 0.001), whereas serum triiodothyronine (T₃) and FT₃I levels were both reduced by 7% (p < 0.05). In the patients, serum TSH was reduced significantly (p < 0.001). No significant differences were observed between patients and normals in regional or total bone mineral content or bone mineral density levels, apart from 20% higher lumbar bone mineral content among the premenopausal patients (p < 0.05). Surprisingly, the mean serum calcium level was slightly elevated (2.38 ± 0.08 vs 2.33 ± 0.07 nmol/l, p < 0.001), serum phosphate decreased (1.13 ± 0.19 vs 1.23 ± 0.16 mmol/l, p < 0.01) and 24-h renal calcium excretion was reduced by 19% (p < 0.05). No changes were observed in serum magnesium, intact parathyroid hormone or calcitriol. The biochemical markers of bone resorption (serum carboxyterminal telopeptide of type I collagen, renal excretion of hydroxyproline, pyridinoline and deoxypyridinoline) and formation (serum levels of carboxyterminal propeptide of type I procollagen, osteocalcin and total and bone alkaline phosphatase) were similar in the two groups. We conclude that long-term thyroxine replacement therapy in primary hypothyroidism does not exert a negative effect on bone mass or alter bone turnover. Bente L Langdahl, University Department of Endocrinology and Metabolism, Aarhus Amtssygehus, Tage Hansensgade 2, DK-8000, Aarhus C, Denmark