Aims: To evaluate the effect of subclinical and overt hypothyroidism on mean platelet volume (MPV) and platelet distribution width (PDW). Study Design: Cross sectional study. Place and Duration of Study: Outpatient Clinic of Diabetes, Metabolism and Endocrinology Unit in internal medicine department and clinical pathology, Tanta University, Egypt in a period between June 2018 to June 2019. Methodology: We tested 250 subjects; 50 healthy control and 200 hypothyroid patients (all were female; age ≥18 years old age) without any medical history, then patients were subdivided into newly diagnosed group and another group on l-thyroxin treatment. Platelet parameters (MPV, PDW) were assessed in all of them. Results: MPV & PDW is increasing in hypothyroid patients than control group (8.93 ± 0.62), more in overt group (10.88 ± 0.57) than subclinical group (10.02 ± 0.55). No significant reduction in MPV and PDW in patients on l-thyroxin treatment which suggest that l-thyroxin treatment couldn’t produce difference till patients reaching their target level being euthyroid. Conclusion: We recommend the usefulness of MPV and PDW as inexpensive markers of platelet activation in diagnostic work-up of athero-thrombotic complications risk in patients with subclinical and overt hypothyroidism.