T-IR- Study to Understand the Effects of Testosterone and Estrogen on the Body's Response to the Hormone Insulin

The investigators will examine the effects of testosterone on insulin sensitivity and body composition in men. This study may lend greater insight into the increased risk of diabetes evident in men with low circulating levels of testosterone. Three drugs will be used in this study: acyline, given by injection; testosterone (T) gel that is applied to the skin; and letrozole, which is an oral drug that blocks the conversion of androgens (male hormones) to estrogens (female hormones). Acyline inhibits the production of luteinizing hormone (LH) and follicle stimulating hormone (FSH). When acyline stops the production of these hormones, it blocks the signal from the brain that stimulates the testicles to make testosterone. Adding testosterone to acyline will restore physiologic levels of testosterone in some study participants. One group of men will receive T gel with letrozole, an aromatase inhibitor; these men will have normal levels of testosterone but low levels of estrogen in the blood. This design will enable determination of the respective metabolic effects of testosterone and estrogen.