Objective: To assess the prevalence of vitamin B12 deficiency in patients of primary hypothyroidism Material and Method: 110 patients of primary hypothyroidism and 110 healthy control were evaluated for B12 deficiency. TPO were also evaluated in B12 deficient hypothyroid person. Result: Prevalence of vitamin B12 deficiency was 45.2% in my study. There was a statistically significant decrease in vitamin B12 level as compared to control group. (196.65±74.33 µg/ml vs 344.918±73.946µg/ml). 12.7% of hypothyroid patient have vitamin B12 level less than 100µg/ml. 45.25 of vitamin B12 deficient hypothyroid patients have TPO positive. But there is no correlation between level of vitamin B12 and TPO. Conclusion: Because of high prevalence of vitamin B12 deficiency and confusing sign and symptoms screening of vitamin B12 should be done at the time of diagnosis of primary hypothyroidism. And it should be repeated thereafter at periodic interval.