Fine adjustments in thyroxine replacement and its effect on bone metabolism.

We studied 10 postmenopausal women with Hashimoto's hypothyroidism treated with varying doses of L-thyroxine replacement. Each patient received incremental doses of L-thyroxine sufficient to achieve subclinical hypothyroidism, euthyroidism, and subclinical hyperthyroidism as determined by normal total serum thyroxine levels (80-160 micrograms/L) and serum TSH concentrations greater than 3.5, 0.3-3.5, and less than 0.3 mU/L, respectively. Metabolic parameters of bone turnover (including serum bone Gla-protein, serum alkaline phosphatase, serum procollagen 1, and serum carboxytelopeptide) were assessed once steady state was achieved, and measurements were compared to 10 healthy controls matched for age and years since the menopause. Our findings suggest that overzealous thyroxine replacement producing subclinical hyperthyroidism may result in an increase in bone turnover as reflected by elevated serum carboxytelopeptide concentrations.