BACKGROUND: The effect of thyroid hormones on cardiovascular system, lipid profiles and other systems were documented in various studies. But the finding of elevated mean platelet volume in hypothyroid individual’s gains enormous significance recently because of indirect effects on management and outcome of the hypothyroidism. OBJECTIVES: 1. To assess whether mean platelet volume levels are elevated in subclinical and overt hypothyroidism. 2. To assess whether the severity of hypothyroidism correlates with the mean platelet volume. MATERIALS AND METHODS: The study was done among 50 hypothyroid and equal euthyroid individuals as prospective observational study at Government Sivagangai Hospital, in medicine outpatient department during June 2018 to December 2018. After getting the IEC clearance and patient informed consent the study participants were subjected to Blood samples were collected to measure blood sugar, urea, creatinine and serum cholesterol. Total T3, total T4 and TSH were measured using the chemiluminescence method. RESULTS: The mean platelet volume was significantly higher in the overt and subclinical hypothyroidism group when compared with the euthyroid group. Also the mean platelet volume was greater in the overt hypothyroidism group when compared with the subclinical hypothyroidism. The p value was <0.001 which signifies that the difference in MPV among the three groups is statistically significant and not due to chance.The correlation coefficient for T3 and MPV is –0.73 whereas for T4 and MPV it is –0.69 indicating that both T3 and T4 are inversely proportional to MPV. Hence with decrease in levels of T3 and T4 there is a proportionate increase in MPV. The correlation coefficient for TSH and MPV is 0.751 indicating that there is a strong positive correlation. Thus MPV values increase with increase in TSH. CONCLUSION: Our study has shown that hypothyroidism increases platelet activation which can be seen from the increased MPV values.