Background: Body weight (BW) and age have been shown to affect the dose of levothyroxine (LT 4 ) that results in normalization of serum thyroid-stimulating hormone (TSH) in hypothyroid patients. Our objective was to determine whether gender, menstrual status, and ideal BW (IBW) also affect the LT 4 dose required to achieve a serum TSH within the normal range. Methods: We retrospectively reviewed the charts of patients being treated for primary hypothyroidism who had TSH values within a normal range. We selected patients aged 18-85 years who were taking LT 4 without any confounding medications, and who had no serious chronic conditions. Their LT 4 doses, referred to here as LT 4 dose requirements, based on both BW and IBW were documented. The relationship between gender, menstrual status, age, serum TSH concentrations, and the degree of overweight on LT 4 dose requirements were determined using multivariate analyses. Results: Women were significantly more overweight than men (ratio of BW/IBW was 1.35 for women vs. 1.17 for men, p < 0.0001). LT 4 requirements based on BW did not differ by gender when age was included in the model. However, when degree of overweight was also included, men required lower LT 4 doses than both premenopausal women (1.34 mg/kg vs. 1.51 mg/kg, p ¼ 0.038) and menopausal women (1.34 mg/kg vs. 1.49 mg/ kg, p ¼ 0.023). When examining IBW using a model incorporating age, men also required lower LT 4 doses than both premenopausal women (1.64 mg/kg vs. 1.92 mg/kg, p ¼ 0.0033) and menopausal women (1.64 mg/kg vs. 1.90 mg/kg, p ¼ 0.0024). Serum TSH concentrations were not significantly different in any of the gender groups. There was no relationship between serum TSH and either age or BW. The initial serum TSH concentration was by design with the normal range, but the concentration within that range was not a significant predictor of the LT 4 replacement dose in any of the models. Conclusion: In contrast to previous studies suggesting that age affects LT 4 replacement requirements, we found that age-based differences in doses are secondary to differences in BW and gender. In addition, in contrast to prior studies showing that lean body mass, but not gender, affected LT 4 dose, we instead found a significant impact of gender. Gender-based differences in dose requirement only became apparent either when IBW was used to correct for the dose or when degree of overweight was included in the model. Gender differences in LT 4 dose requirement exist, but are masked unless gender-based differences in degree of overweight are also considered.