Vitamin D deficiency is a global health problem, its role as an immune modulator has been recently emphasized. The evidence is increasingly pointing towards vitamin D significant role in reducing the incidence of autoimmune diseases. However, at this time the research on its role in autoimmune and thyroid disease is not conclusive.

We aimed to examine the relationship between hypothyroidism and vitamin D deficiency and to clarify the relation between serum calcium levels with hypothyroid disease.

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"Vitamin D deficiency is a global health problem. (1) Over a billion people worldwide are vitamin D deficient or insufficient. (1) Yet no international health organization or governmental body has declared a health emergency to warn the public about the urgent need of achieving sufficient vitamin D blood levels. (2) Understanding of the role of vitamin D has been evolving since its discovery in the early 20th century from being a simple vitamin to a steroid pro-hormone. (3) It has been recognized to be involved in various immune functions as well as bone and muscle development. (3) Vitamin D deficiency has been shown to be associated with autoimmune diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), inflammatory bowel disease (IBD), multiple sclerosis (MS) and type 1 diabetes (T1DM), and that vitamin D supplementation prevents the onset and/or development of these autoimmune diseases. (4) Furthermore, it was reported that patients with Hashimoto’s thyroiditis, an autoimmune thyroid disease had lower vitamin D levels. (5) Vitamin D plays an essential role in calcium homeostasis and the development and maintenance of the skeleton. (6) It is recognized as the sunshine fat soluble vitamin. Exposure to ultraviolet B light (290–320 nm) are the main source of vitamin D. (7) In the classical endocrine pathway, vitamin D enters the circulation attached to a D-binding protein, is first hydroxylated in the liver to 25(OH) D and then in the kidney to form the active metabolite, 1, 25 dihydroxy vitamin D (1, 25-(OH)2 D) or calcitriol. (8) Serum 25(OH) D, the most abundant circulating precursor of active vitamin D, is the most widely accepted indicator of vitamin D status and reflects combined contributions from cutaneous synthesis. (9) Serum 25(OH)D has a half-life of approximately two to three weeks, in contrast, 1,25-(OH)2D has a short circulating half-life and is tightly regulated over a narrow range by parathyroid hormone, calcium and phosphate. (9) Serum 1,25-(OH)2D is not a good measure of vitamin D status since a decrease may not occur until vitamin D deficiency is severe. (10) Levels of 25(OH)D 30 to 32 ng/ml is considered to be sufficient, but levels of 20 to 29 ng/mL is insufficient and if it is less than 12 ng/ml is considered an evidence of severe vitamin D deficiency. (11) Importantly, both vitamin D and thyroid hormone bind to similar receptors called steroid hormone receptors. A different gene in the Vitamin D receptor was shown to predispose people to autoimmune thyroid disease including Graves’ disease and Hashimoto’s thyroiditis. For these reasons, it is important for patients with thyroid problems to understand how the vitamin D system works. (12) Vitamin D mediates its effect though binding to vitamin D receptor (VDR), and activation of VDR-responsive genes. While VDR gene polymorphism was found to associate with autoimmune thyroid diseases (AITDs). (12)

The purpose of this study was to examine the relationship between hypothyroidism and vitamin D deficiency and to clarify the relation between serum calcium levels with hypothyroid disease."