Diabetes mellitus (DM) and thyroid dysfunction (TD) are the two most common endocrine disorders in clinical practice. Various studies have found that diabetes and thyroid disorders mutually influence each other and both disorders tend to coexist. The objective of this study was to investigate the prevalence of thyroid dysfunction in patients with type 1 and type 2 diabetes mellitus. Methods: This is a prospective descriptive study which was conducted on 186 patients of diabetes mellitus who

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... nded the outpatient diabetic clinic of Sri Ramakrishna Hospital, Coimbatore, Tamil Nadu from January 2015 to June 2015. All patients underwent a clinical and laboratory evaluation. The correlation of prevalence of thyroid disorder with gender distribution, age distribution, HbA1C, duration of diabetes, hypertension, family history of thyroid disorder, and dyslipidemia was then done. The observations and interpretations were recorded and results obtained were statistically analyzed. Results: The prevalence of thyroid dysfunction in diabetic patients was found to be 21.5%, in which the most common thyroid disorder being the subclinical hypothyroidism(12.4%) followed by subclinical hyperthyroidism (6.5%) which was followed by primary hyperthyroidism (2.7%) and primary hypothyroidism (0%).Prevalence of thyroid disorders was found to be more in males(n=95) than in females(n=91), highest in the age group of <50 years, more in patients with HbA1c values ≥7, i.e. in uncontrolled diabetes. Conclusions: The prevalence of thyroid dysfunction among type 2 Diabetes Mellitus patients is very high (21.5 %) with subclinical hypothyroidism is being most common. Screening of thyroid dysfunction should be done in all diabetic patients especially in patients with poor diabetic control.