Somewhat ironic that we see this very detailed explanation of why "active" vitamin D is low in HYPERthyroid people, while the most common story here is of HYPOthyroid people having low vitamin D!
As so often, the work was done in mice. Be good to know if the same is true in humans.
Endocrinology. 2013 Jan 10. [Epub ahead of print]
Thyroid Hormones Decrease Plasma 1a,25-Dihydroxyvitamin D Levels Through Transcriptional Repression of the Renal 25-Hydroxyvitamin D3 1a-Hydroxylase Gene (CYP27B1).
Kozai M, Yamamoto H, Ishiguro M, Harada N, Masuda M, Kagawa T, Takei Y, Otani A, Nakahashi O, Ikeda S, Taketani Y, Takeyama KI, Kato S, Takeda E.
Departments of Clinical Nutrition (M.K., H.Y., M.I., M.M., T.K, Y.Takei., A.O., O.N., S.I., Y.Taket., E.T.) and Nutrition and Metabolism (N.H.), Institute of Health Biosciences, University of Tokushima Graduate School, Tokushima 770-8503 Japan; and Institute of Molecular and Cellular Biosciences (K.T., S.K.), the University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan.
The primary determinant of circulating 1a,25-dihydroxyvitamin D (1,25[OH](2)D) levels is the activity of 25-hydroxyvitamin D-1a-hydroxylase (cytochrome P450 27B1 [CYP27B1]) in the kidney. Hyperthyroid patients have been reported to have low levels of plasma 1,25(OH)(2)D. However, the detailed mechanism of thyroid hormone action on vitamin D metabolism is still poorly understood. The present study determined whether renal CYP27B1 gene expression was negatively regulated by thyroid hormones. T(3)-induced hyperthyroid mice showed marked decreases in plasma 1,25(OH)(2)D levels and in renal expression of CYP27B1 mRNA but no changes in plasma concentrations of calcium, PTH, or fibroblast growth factor-23. In addition, we observed that T(3) administration significantly decreased plasma 1,25(OH)(2)D and renal CYP27B1 mRNA levels that were increased by low-calcium or low-phosphorus diets and induced hypocalcemia in mice fed a low-calcium diet. Promoter analysis revealed that T(3) decreases the basal transcriptional activity of the CYP27B1 gene through thyroid hormone receptors (TRa and TRß1) and the retinoid X receptor a (RXRa) in renal proximal tubular cells. Interestingly, we identified an everted repeat negative thyroid hormone response element (1a-nTRE) overlapping the sterol regulatory element (1a-SRE) and the TATA-box -50 to -20 base pairs from the human CYP27B1 gene transcription start site. Finally, we established that CYP27B1 gene transcription is positively regulated by SRE-binding proteins and that a T(3)-bound TRß1/RXRa heterodimer inhibits SRE-binding protein-1c-induced transcriptional activity through the 1a-nTRE. These results suggest that transcriptional repression of the CYP27B1 gene by T(3)-bound TRs/RXRa, acting through the 1a-nTRE, results in decreased renal CYP27B1 expression and plasma 1,25(OH)(2)D levels.