Recombinant human TSH (rhTSH) has been proposed as an alternative method to the withdrawal of thyroid hormones in the follow-up of differentiated thyroid cancer. The aim of the present study was to evaluate the influence of several demographic and anthropometric parameters [age, body weight, height, body mass index, and body surface area (BSA)] on serum peak TSH levels after rhTSH administration. rhTSH was administered to 112 patients with differentiated thyroid carcinoma according to the conventional two-dose schedule (0.9 mg/d). Serum TSH levels were measured 24 h before and after the first administration of rhTSH, and then 24, 48, and 72 h after the second administration of rhTSH. In one severely obese patient, serum peak TSH values did not reach a valid stimulation range. Serum peak TSH levels were negatively related to body weight (r = -0.69; P < 0.0001), body mass index (r = -0.51; P < 0.0001), and BSA (r = -0.72; P < 0.0001). In a multivariate regression analysis including demographic and anthropometric variables, only BSA was independently associated to serum peak TSH concentrations (standardized beta coefficient = -0.721; P < 0.0001). In conclusion, body size seems to influence serum peak TSH levels after rhTSH administration. Future studies should evaluate the possibility of using personalized rhTSH doses, adjusted in relation to BSA.