the relationship between thyroid hormones, vitamin d and fasting blood glucose level during summer and winter in nondiabetic adults

background and aims: 25-hydroxy vitamin d3 is recognized to be an essential element for healthy body; however, its deficiency has been identified as a risk factor for thyroid disease, diabetes mellitus and other autoimmune diseases and infectious diseases. thyroid disease is common in the general population that its prevalence increases proportional to age. subjects with poorly controlled diabetes, with or without its complications, may experience some changes in their thyroid function tests. the present study aimed to evaluate the association between thyroid hormones, vitamin d3 and fasting blood glucose (fbs) level during summer and winter in nondiabetic adults. materials and methods: a total of 1093 caucasian adults without diabetes were chosen for study. the serum levels of 25-hydroxy vitamin d3, thyroxin (t4), thyroid stimulating hormone (tsh) and fbs were measured in these samples and the association of these factors during summer and winter were evaluated. results: the study results demonstrated a significant association in t4 and tsh with 25-hydroxy vitamin d3. the significantly lower serum level of t4 and no significant difference in tsh level were determined in impaired fasting glucose subjects as compared to normal fasting glucose subjects. no significant association was detected between serum level of 25(oh) d3, tsh, t4 and fbs in winter and summer.     conclusions: 25-hydroxy vitamin d3 may play a role on thyroid hormone regulation. in addition, thyroid hormones can affect fbs level. as a result, lifestyle and diet may have possible effects on thyroid hormones and blood glucose regulation.