Hundreds of different hormones have been discovered in animals. The human body alone contains more than 100 different hormones. These hormones are secreted by endocrine glands, also known as ductless glands. Examples of endocrine glands include the hypothalamus, pituitary gland, pineal gland, thyroid, parathyroid, thymus, adrenals, pancreas, ovaries, and testes. Hormones are secreted from these glands directly into the bloodstream. They then travel to target tissues and regulate digestion, growth, maturation, reproduction, and homeostasis (maintaining the body's chemical balance).
If testosterone deficiency occurs during foetal development, then masculinisation of the foetus will fail to occur normally and this may give rise to disorders of sex development. If testosterone deficiency occurs during puberty, a boy’s growth may slow and no growth spurt will be seen. The child may also fail to develop full sexual characteristics (hypogonadism) associated with men undergoing puberty, including development of pubic hair, growth of the penis and testes and deepening of the voice. Around the time of puberty, boys with too little testosterone may also have less than normal strength and endurance, and their arms and legs may continue to grow out of proportion with the rest of their body.
Genitals – Testes will lessen quite significantly in size. The production of testosterone and sperm is also greatly reduced. Penile size will also likely diminish. Sexual function will decrease, but the extent to which performance is affected is unpredictable. Erections may still continue, but will probably be less frequent, and not last as long, and in some cases may not be possible. Ejaculate will lessen, probably to the point of only producing a very small, clear discharge as a result of the prostate and the associated structures responsible for semen production being impeded. It is important to note, however, that the ability to orgasm is not dependent on either an erection, nor ejaculate. Anecdotally speaking, many transgender females report greater satisfaction with their orgasms after these changes occur in their genitals.