Endocrine studies are especially important in patients complaining erectile dysfunction with suggestive symtoms and signs, such as loss of libido, small testicles, and abnormal secondary male characteristics. The endocrine studies include serum testosterone and prolactin in screening for hormonal dysfunction. When suggestive abnormalities are found, full evaluation of the hypothalamic-pituitary-gonadal axis, including LH, FSH, estradiol, T3 and T4 binding capacity should be undertaken. The assays for FSH and LH often do not distinguish normal and subnormal concentrations, thus stimulating tests utilizing clomiphene citrate and Gn-RH have been developed. A dynamic tests using the response to TRH or chlorpromazine have been devised to try separate primary pituitary tumors from the other causes of hyperprolactinemias but the sensitivity and specificity values have not been carefully worked out. The endocrine tests to evaluate the hypogonadism and hyperprolactinemias are important because they can easily be treated with excellent results in a high percentage of cases and these diagnoses may lead to the recognition of a disorder that could require treatment. However, these laboratory tests have variations in normal values depending on the laboratories, patient's age and general condition, blood sampling time, and threshold in some disease. The interpretation of results in many different situations especially after stimulation test is very difficult.