Bone mass, bone turnover and body composition in former hypothyroid patients receiving replacement therapy

Aarhus Bone and Mineral Research Group, University Department of Endocrinology and Metabolism, Denmark.

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 mmol/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.

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