Objective To investigate the relationships among free triiodothyronine( FT3 ), free thyroxine (FT4 ), and thyroid-stimulating hormone( TSH) in both plasma and breast milk of patients with thyroid diseases. Methods A total of 102 female subjects with hyperthyroidism(GD), normal thyroid function(NC), and Hashimoto′s hypothyroidism(HT or hypothyroidism)were enrolled. Their plasma and breast milk were collected for measurement of FT3 and FT4 , and TSH. Meanwhile, 11 infants of patients with hyperthyroidism and another 11 infants of patients with hypothyroidism were selected, blood FT3 , FT4 , and TSH content were determined during lactating period and 2 months after lactation. Results (1) FT3 and FT4 contents in breast milk among 3 groups were different[(1. 48 ± 0. 81), (7. 79 ± 3. 56), and (0. 77 ± 0. 42)pg/ ml; (2. 94 ± 1. 43), (14. 78 ± 7. 40), and (1. 51 ± 0. 40)pg/ ml, P<0. 05], TSH in breast milk was similar between hyperthyroidism and normal groups(P>0. 05). (2) FT3 ratio of breast milk to plasma of the hyperthyroidism group was different to other 2 groups(0. 42 ± 0. 04 vs 0. 35 ± 0. 03, 0. 36 ± 0. 03, P<0. 05), but no difference existed in FT4 and TSH among 3 groups(both P>0. 05). (3)Blood FT3 , FT4 , and TSH contents from infants of patients with hyperthyroidism and hypothyroidism were different, both during lactating period and 2 months after lactation[(5. 06 ± 1. 76)vs (6. 51 ± 2. 23)pg/ ml, (17. 39 ± 2. 78)vs (19. 87 ± 3. 26)pg/ ml, (1. 34 ± 1. 33)vs (0. 74 ± 0. 78)mIU/ L; (1. 43 ± 0. 74)vs (1. 83 ± 0. 91)pg/ ml, (4. 28 ± 1. 55)vs (5. 00 ± 1. 75)pg/ ml, (6. 48 ± 2. 70) vs (5. 49 ± 2. 39) mIU/ L; all P<0. 05]. (4) FT3 and FT4 contents were positively correlated in plasma and breast milk(all P<0. 05), while TSH contents were positively correlated only in hypothyroidism group(P<0. 05). Conclusion FT3 , FT4 , and TSH in blood and breast milk are correlated.