Each sperm produced by the testes takes about seventy-two days to mature and its maturity is overseen by a complex interaction of hormones. The scrotum has a built-in thermostat that keeps the testes and sperm at the correct temperature. It may be surprising that the testes should lie in such a vulnerable place outside the body, but it is too hot for them inside. Spermatogenesis requires a temperature that is three to five degrees Fahrenheit below body temperature. If it becomes too cool on the outside, the cremaster muscle will contract to bring the testes closer the body for warmth.
Testosterone is transported in blood bound to a carrier protein, called either testosterone binding globulin (TeBG) or, more commonly, sex hormone binding globulin (SHBG). In healthy men, only about 2% of the circulating testosterone is unbound, with 44% bound to SHBG and 54% bound to serum albumin. The protein-bound testosterone is protected from metabolism in the liver and provides an easily accessible pool of hormone, as the testosterone readily dissociates from its binding protein. Levels of SHBG in plasma are regulated by androgens, oestrogens and thyroid hormones. In healthy men, SHBG levels are fairly constant, but may need to be considered when steroid replacement therapy is used. Plasma testosterone concentrations are around 9–41 nmol/L in healthy men and 1–3 nmol/L in women. Plasma testosterone tends to decrease with age in men. Testosterone is metabolized in the liver, mostly to form androsterone and aetiocholanolone, which are excreted in urine.
As men and women age their testosterone levels diminish. Testosterone is known as the Hormone of Desire . In middle-aged men, low testosterone symptoms (Low T) or Andropause start to appear. Some of the most profound symptoms of low testosterone are - feeling extremely tired all day, unexpected weight gain especially increased belly fat, loss of muscle or muscles getting flabby, low sex drive or a complete lack of desire, softer erections or complete erectile dysfunction, inability to sleep or outright insomnia, memory and hair loss, bursts of anger, social withdrawal and depression.