Background: Cardiovascular performance is well-known to be compromised due to low level of thyroid hormone levels in terms of reduced cardiac performance, reduced chronotropy, and increased systemic vascular resistance. Persistent subclinical hypothyroidism (SCH) is also a known risk factor for cardiomyopathy and heart failure. Yet, the direct effect of hypothyroidism on the heart, independent of its effect on loading function remains to be elucidated. In this study, we wanted to see the effect of hypothyroidism correction within 6 months on cardiac performance as measured by pre-defined echocardiographic parameters. Objectives: To find out the change in left ventricular systolic and diastolic function as well as left ventricular mass index (LVMI) with treatment of primary hypothyroidism with levothyroxine replacement in 6 months time frame and to detect reversibility, if present at all, of E/E'(E—early transmitral diastolic velocity, E'– tissue Doppler early diastolic velocity), left ventricular ejection fraction (LVEF) and LVMI after patient become biochemically euthyroid. Results: A total of 51 cases of primary hypothyroidism were selected after excluding other comorbidities ranging from age 12–67 years, to undergo assessment of E/E', LVEF, and LVMI echocardiographically at first OPD visit (t = 0) and at 6 months. Age- and sex-matched healthy controls were taken for comparison. Meanwhile, they were being treated with levothyroxine supplementation at an appropriate dosage to establish a euthyroid status during this period. They were monitored at regular intervals. At 6 months all of them became euthyroid biochemically and their echo parameters were compared. The mean age of the study group was 44.57 ± 10.53 years among cases. In case arm, 23 out of 51 (45.1%) patients were female and 28 (54.9%) patients were male. Male–female ratio was ~ 1.2:1. The mean thyroid stimulating hormone (TSH) at t = 0 (mean ± SD) was 19.90 ± 23.90 mIU/L. The mean FT4 at t = 0 (mean ± SD) of patients was 1.12 ± 0.15 ng/dL. The association of E/E' at t = 0 in case vs control group was statistically significant (p < 0.0001). The association of EF at t = 0 in case vs control group was statistically significant (p = 0.0068). The association of LVMI at t = 6 months vs at t = 0 group was statistically significant (p = 0.0339). There was significant improvement in E/E' (p = 0.0010), EF (p < 0.0001), in LVMI (p < 0.0001) after 6 months. The positive correlation between E/E' at t = 6 months vs TSH at t = 6 months was not statistically significant (p = 0.48). Negative correlation between EF at t = 6 months vs TSH at t = 6 months was not statistically significant (p = 0.57). The positive correlation between LVMI at t = 6 months vs TSH at t = 6 months was not statistically significant (p = 0.89). Negative correlation between TSH at t = 6 months vs FT4 at t = 6 months was not statistically significant (p = 0.348). Conclusion: Primary hypothyroidism is associated with diastolic dysfunction, left ventricular systolic dysfunction, and abnormal left ventricular mass. The most affected age group is the 5th decade or above. Diastolic dysfunction is more severe than systolic dysfunction. Levothyroxine supplementation can potentially revert these dysfunctions. It takes at least 6 months for cardiac function to reverse. Early diagnosis of hypothyroidism and levothyroxine supplementation can prevent potentially dangerous complications of cardiovascular system including heart failure