Background
 Vitamin D deficiency is a global health problem, and its role as an immune modulator has been recently tightened. The main role of vitamin D is to maintain calcium and phosphorus homeostasis, thus maintaining bone health. Recent evidence has shown that vitamin D may play a role in a variety disorders such as endocrine diseases, adrenal diseases, polycystic ovary syndrome and particular type 1 diabetes, type 2 diabetes. We evaluated the association between thyroid disorders (subclinical or overt hypothyroidism and subclinical or overt hyperthyroidism) and serum vitamin D3 level in the Indian population and its association with ionized calcium.
 Methods
 The cross-sectional study, included patients who visited Shree Krishna Hospital, Karamsad for thyroid evaluation or follow-up. We conducted this study involving a total of 84 individuals with normal thyroid function, were recorded as control. The cases group included 75 patients with hypothyroidism (overt or subclinical) and 10 patients with hyperthyroidism (overt or subclinical). Serum levels of vitamin D3, thyroid function and calcium ionized were measured in all adult subjects. Deficiency of vitamin D is defined as the level of vitamin D3 in the serum less than 50 nmol/L. Vitamin D insufficiency is defined as serum vitamin D3 level between 50 - 75 nmol/L. Vitamin D sufficiency is defined as serum Vit D3 level >75 nmol/L.
 Result 
 The prevalence of vitamin D deficiency in subclinical and clinical hypothyroidism cases (36 of 40, 90%) was significantly higher than that observed in healthy individuals (0 of 40, 0%). The prevalence of vitamin D deficiency in subclinical and clinical hyperthyroidism cases (4 of 40, 10%). The prevalence of vitamin D insufficiency in subclinical and clinical hypothyroidism cases (24 of 62, 38.7%) was significantly lower than that observed in healthy individuals (controls) (35 of 62, 56.5%).The prevalence of vitamin D insufficiency in subclinical and clinical hyperthyroidism cases (3 of 62, 4.8%). The prevalence of vitamin D sufficiency in subclinical and clinical hypothyroidism cases (15 of 67, 22.4%) was significantly lower than that observed in healthy individuals (controls) (49 of 67, 73.1 %). The prevalence of vitamin D sufficiency in subclinical and clinical hyperthyroidism cases (3 of 67, 4.5%).
 Conclusion 
 Vitamin D deficiency was associated with hypothyroidism especially clinical hypothyroidism. Low serum levels of vitamin D3 were associated with high serum TSH levels.
 Keyword: Vitamin D3, calcium ionized, hypothyroidism and hyperthyroidism.