OBJECTIVE To explore the relationship between maternal milk and serum thyroid hormones in patients with thyroid-related diseases. METHODS Serum and breast milk samples were collected from 56 breastfeeding mothers. Milk and serum free triiodothyronine (FT3), free thyroxine (FT4), triiodothyronine(T3), thyroxine (T4), and thyrotrophin (TSH) were determined, and T3/T4 was calculated. Using the serum thyroid hormones as the independent variables and milk thyroid hormones as the dependent variables, we performed linear regression analysis. RESULTS The milk FT3, FT4, T3, T4, TSH, and T3/T4 were (2.30 ± 0.82) pg/ml ,(0.45 ± 0.26) ng/dl, (0.35 ± 0.20) ng/ml, (2.96 ± 1.55) Μg/dl, (0.12 ± 0.08) ΜU/ml, and 0.12 ± 0.04, respectively. Milk FT3 (r = 0.778, P = 0.000), T3 (r = 0.603, P = 0.000), T4 (r = 0.485, P = 0.004), and TSH (r = 0.605, P = 0.000) concentrations were positively correlated with those in serum. CONCLUSIONS Thyroid hormones are present in human milk and are positively correlated with those in serum.