Libido is controlled by the brain and not the testes or ovaries, although diseases of these glands can certainly have an adverse effect on libido as they do not respond to stimuli from the brain.
To enjoy and be successful in achieving sexual intercourse, both partners must be relaxed, secure and comfortable. Psychological stress of any sort will reduce sexual desire. Examples can be as wide ranging as worries about job, money, pregnancy, discovery (“will the children come in?”), the relationship itself or disease.
Many psychiatric conditions, but particularly depression, will remove any desire for sex. Difficulty in sleeping, loss of interest in other activities and poor self-esteem are other signs of depression.
Failure of any major organ of the body (e.g. heart, liver, kidney) or any other serious disease will affect the normal hormonal and chemical balances, as well as causing stress and anxiety. Sex then becomes something to be remembered rather than sought.
Disease, infection, tumour (e.g. Fröhlich syndrome), injury or cancer of the pituitary gland under the centre of the brain will affect libido. This tiny gland is the conductor of the gland orchestra in the body, and is itself directly controlled by the brain. If for one of these reasons it does not produce the necessary hormones to stimulate the testes or ovaries, they will not release the appropriate sex hormones (testosterone and oestrogen) to allow appropriate sexual responses. Rarely the pituitary gland may become over active, and over stimulate the sex glands to drain them of their hormones.
The part of the brain controlling the pituitary gland can itself be affected by a stroke, bleeding, injury, tumour, cancer or abscess. Parkinson’s disease and other degenerative conditions of the brain will both reduce desire and ability.
In men, any disease that reduces the production of testosterone (male hormone) in the testes will reduce libido. Examples include infections (orchitis), tumours (e.g. cancer), cysts and torsion (twisting to cut off the blood supply). Other causes of low libido in men include enlargement of the prostate gland and poorly controlled diabetes mellitus.
Women find that their libido varies during the month, usually being highest at the time of ovulation (when they are most likely to get pregnant) half way between the start of one period and the next, and lowest during a menstrual period.
Pregnancy also lowers libido for its duration and breastfeeding has a similar effect on the hormones.
Other causes of low libido in women include tumours or cysts of the ovary. During the menopause, when there is a lack of oestrogen, sex may be uncomfortable as well as undesirable.
Numerous drugs (legal, illegal and prescribed) can reduce libido. Examples include alcohol, heroin, marijuana, steroids, antihistamines (e.g. cold preparations), benzodiazepines (e.g. diazepam, oxazepam) and fluid pills, as well as some medications used to treat depression (tricyclics) and decreased high blood pressure (beta blockers).