© 2016 Polskie Towarzystwo Kardiologiczne.Background: Subclinical hypothyroidism (SCH) is a common disorder which has adverse cardiovascular effects. Epicardial adipose tissue (EAT), a novel marker of cardiovascular risk, is increased in SCH. Aim: We aimed to investigate whether L-thyroxine treatment can reverse the thickening of EAT in SCH. Methods: Forty-four patients with SCH and 42 euthyroid control subjects were included. EAT thickness was measured using transthoracic echocardiography at baseline and after restoration of the euthyroid status with 3 months of L-thyroxine treatment. Results: At baseline, mean EAT thickness was signifcantly greater in the SCH group when compared to the control group (6.3 ± 1.7 mm vs. 4.1 ± 0.9 mm, respectively, p < 0.001). There was a signifcant positive correlation between baseline serum thyroid stimulating hormone (TSH) level and EAT thickness in the SCH group. There was a signifcant reduction in mean EAT thickness in response to L-thyroxine treatment (6.3 ± 1.7 mm vs. 5.1 ± 1.4 mm, p < 0.001). The decrease in EAT thickness after L-thyroxine treatment when compared to baseline (DEAT) signifcantly correlated to the difference in TSH levels before and after treatment (DTSH; r = 0.323; p = 0.032). Conclusions: Epicardial adipose tissue thickness is increased in patients with SCH. This thickening was alleviated with restoration of the euthyroid status with L-thyroxine treatment in our study population of predominantly male, relatively old subjects with greater baseline EAT thickness.