The aim of this survey was to assess the correlation between leptin and insulin sensitivity (IS) in cases of diffuse toxic goiter. This is a descriptive study on patients with diffuse toxic goiter (DTG) assessing their body mass index (BMI), serum leptin concentrations, circulating insulin (area under the curve (AuC) of insulin), average insulin level, thyroid hormones, thyroid stimulating hormone (TSH), glycemia and IS (using a hyperinsulinemic-euglycemic clamp and the homeostasis model for assessment of insulin resistance (HOMA-IR) before and after euthyroidism induced with metimazol. The average patient age was 35 years old (range 31-40 years), height was 157 cm (range 151-160 cm), glycemia was 4.3 +/- 0.3 mmol/L and TSH 0.1 +/- 0.1 microU/mL. Average leptin level was 11.3 +/- 2.8 ng/dL, the average insulin level was 10.13 +/- 3.7 mIU/mL and the AuC for insulin was 50.6 +/- 18 microIU x min/mL. No correlation was found between leptin and BMI, thyroid hormones and glycemia. While controlling for the BMI effect, a correlation was found between leptin and TSH (r = -0.77, p = 0.042), as well as between leptin and insulinemia (r = 0.93, r2 = 0.86, p = 0.001) independently from the state of thyroid function. There was a tendency for a high correlation between leptin and the insulin AuC (hyperthyroidism: r = 0.89, p = 0.056; euthyroidism: r = 0.99, p = 0.056). A negative correlation was found between IS and the insulin AuC (rho = -0.58, p = 0.18). There was a high tendency for correlation between leptin and IS when the BMI effect (HOMA-IR: r = 0.70, p = 0.12; PHE: r = -0.55, p = 0.26) was taken into consideration. There is a high tendency for a negative correlation between leptin and IS when the BMI effect is controlled. There is a high tendency for a positive correlation between leptin and insulin and TSH.