Vitamin D Metabolites in Serum from Hypoparathyroid Patients Treated with Vitamin D2and la-Hydroxyvitamin D3*

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Abstract
Vitamin D and its metabolites 25-hydroxyvitamin D (25OHD), 24,25-dihydroxyvitamin D [24,25-(OH)2D], 25,26-(OH)2D and 1,25-(OH)2D were measured after separation on high pressure liquid chromatography in sera from 2 hypoparathyroid patients treated with high doses of vitamin D2. Fractions with displacement activities in the competitive protein-binding assays were found which were not detectable in sera from controls not supplemented with vitamin D2. These fractions presumably representing vitamin D2 metabolites, were quantitated separately from the vitamin D3 metabolites. After a change of treatment from mg doses of vitamin D2 to .mu.g doses of 1 .alpha.OHD3, the serum metabolites of vitamin D2 and vitamin D3 were followed from 8-13 mo. During the first 2-3 mo., there was an initial relatively rapid fall in serum vitamin D2 levels and metabolites, followed by a slower decline. High levels of vitamin D2 metabolites were still present after 13 mo. Taking into account the marked preponderance of 25OHD2 to 25OHD3, the relative concentration of (OH)2D3 metabolites were higher than expected, which might indicate a preferential 25OHD3 hydroxylation or alternatively, a more rapid degradation of vitamin D2 metabolites in these patients. The high and sustained release of vitamin D2, presumably from fat stores, more than a year after vitamin D2 ingestion was stopped has obvious clinical implications and should be considered in the long term follow-up of patients shifted from the traditional high doses of vitamin D to the newly synthesized 1.alpha.OHD3 or 1,25(OH)2D3.