Objective To evaluate changes in serum levels of thyroid hormones in psychiatric patients after SGAs treatment, and to determine differences between SGAs monotherapy and polytherapy, as well as differences between patients with or without a history of antipsychotic drugs. Material and Methods A total of 71 patients with baseline thyroid hormone levels within normal reference range were included in this retrospectively study. Results After SGAs treatment, the serum levels of free triiodothyronine (FT3), free thyroxine (FT4), total triiodothyronine (TT3), and total thyroxine (TT4) significantly decreased, while serum thyroid-stimulating hormone (TSH) levels significantly increased (change from baseline: FT3: -0.31 pmol/L, p < 0.001; FT4: -2.71 pmol/L, p < 0.001; TT3: -0.05 nmol/L, p = 0.024, TT4: -12.36 nmol/L, p < 0.001; TSH: 0.46 mIU/L, p < 0.001). One (1.4%) patient had TSH levels higher than 7 mIU/L. Changes in serum levels of TSH and TT3 between patients with or without a history of antipsychotics drug were significantly different (change from baseline: TSH p < 0.001 vs. p = 0.089; TT3 p = 0.013 vs. p = 0.553). Changes in serum TSH levels had a moderate positive correlation with average daily dose of SGAs (p = 0.007, r = 0.318). Conclusions There seemed to be a trend of hypothyroidism in patients after SGAs treatment, but the incidence of hypothyroidism was low. The preliminary results of this study did not show the necessity of regular monitoring on serum thyroid hormone levels in patients with baseline thyroid hormone levels within normal reference range. Objective To evaluate changes in serum levels of thyroid hormones in psychiatric patients after SGAs treatment, and to determine differences between SGAs monotherapy and polytherapy, as well as differences between patients with or without a history of antipsychotic drugs. Material and Methods A total of 71 patients with baseline thyroid hormone levels within normal reference range were included in this retrospectively study. Results After SGAs treatment, the serum levels of free triiodothyronine (FT3), free thyroxine (FT4), total triiodothyronine (TT3), and total thyroxine (TT4) significantly decreased, while serum thyroid-stimulating hormone (TSH) levels significantly increased (change from baseline: FT3: -0.31 pmol/L, p < 0.001; FT4: -2.71 pmol/L, p < 0.001; TT3: -0.05 nmol/L, p = 0.024, TT4: -12.36 nmol/L, p < 0.001; TSH: 0.46 mIU/L, p < 0.001). One (1.4%) patient had TSH levels higher than 7 mIU/L. Changes in serum levels of TSH and TT3 between patients with or without a history of antipsychotics drug were significantly different (change from baseline: TSH p < 0.001 vs. p = 0.089; TT3 p = 0.013 vs. p = 0.553). Changes in serum TSH levels had a moderate positive correlation with average daily dose of SGAs (p = 0.007, r = 0.318). Conclusions There seemed to be a trend of hypothyroidism in patients after SGAs treatment, but the incidence of hypothyroidism was low. The preliminary results of this study did not show the necessity of regular monitoring on serum thyroid hormone levels in patients with baseline thyroid hormone levels within normal reference range.