VITAMIN D METABOLISM IN HYPERTHYROIDISM

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Abstract
The serum concentrations of 25‐hydroxycholecalciferol (25 OH D3), 24,25‐dihydroxycholecalciferol [24,25(OH)2D3] and 1,25‐dihydroxycholecalciferol [1,25(OH)2D3] were measured in twenty‐one patients with untreated hyperthyroidism. Compared with control subjects, 25 OH D3 concentrations were not altered, 24,25(OH)2D3 concentrations were increased, although not significantly and 1,25(OH)2D3 concentrations were decreased (P= 0.01). Following oral carbimazole therapy, 24,25(OH)2D3 concentrations fell (P2D3 concentrations increased (P3 concentrations were unchanged. The altered 1,25(OH)2D3 and 24,25(OH)2D3 concentrations found in hyperthyroidism are probably due to the effects of thyroid hormone on bone and mineral metabolism. Increased serum calcium and phosphate concentrations with secondary hypoparathyroidism result in stimulation of the renal 24‐hydroxylase and suppression of the 1‐hydroxylase enzymes. In addition, serum 24,25(OH)2D3 concentrations were significantly correlated with serum triiodothyronine levels (T3) (r= 0.66, P2D3 concentrations before therapy and serum T3.