What is Testosterone?

By Dr Ananya Mandal, MD

Testosterone is the principle sex hormone responsible for the development of reproductive function in male vertebrates. As a steroid hormone, testosterone is derived form cholesterol.


The majority of the hormone is secreted from the testes, hence the term testosterone. It is also secreted in small amounts by the adrenal gland and by the ovaries in females. Males secrete about ten times more testosterone than females. The structure of testosterone is similar across all mammals. In fish, however, 11-ketotestosterone is produced instead of testosterone.


Testosterone acts as a key facilitator in the growth and development of males and is responsible for the development of the male reproductive organs such as the testes, penis and prostate.

Testosterone also promotes secondary sexual characteristics during puberty such as increased muscle mass, increased bone mass, deepening of the voice, maturation of facial structures, and the growth of armpit, chest and pubic hair. A healthy level of testosterone is also protective against bone disorders such as osteoporosis.


Aside from promoting the physical growth and maintenance of reproductive functions, testosterone also helps maintain a general sense of well-being and energy levels. After the age of 30, testosterone levels typically decline causing a progressive decline in physical energy, fatigue and libido with age.

Reviewed by , BSc

Further Reading

Last Updated: Sep 23, 2014

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