The pituitary gland may function abnormally due to benign tumors, malignant tumors, or infection. Benign tumors are the most common cause of pituitary dysfunction; malignant tumors of the pituitary gland are very rare. Pituitary dysfunction can have a wide range of effects on the body due to overproduction or underproduction of a hormone or due to pressure on and around the pituitary gland. Among those effects are the following:
- Abnormally high or low blood pressure
- Appetite changes
- Rapid weight changes
- Excessive perspiration
- Excessive thirst and frequent urination
- Failure to grow
- Enlarging hand and foot size
- Menstrual changes
- Decreased sex drive
- Abnormal milk discharge
- Impotence
- Infertility
- Fatigue
- Headache
- Intolerance to cold
- Vision changes
- Visual field problems
- Mood changes and depression
- Osteoarthritis
Stalk Effect
Stalk effect explains why prolactin can be elevated when the pituitary tumor is not the source of excess prolactin. The pituitary gland is connected by a stalk to a part of the brain called the hypothalamus. The hypothalamus sends signals to the pituitary gland to produce or to stop producing hormones. Stalk effect is present when prolactin levels are elevated because a nonhormone-producing tumor is present within or above the sella (a depression in the bone under the brain where the pituitary gland resides).
It is thought that nonhormone-producing tumors can elevate prolactin levels for the following reasons. Normally, the hypothalamus sends dopamine (a prolactin inhibitor) through the blood vessels surrounding the pituitary stalk down to the anterior lobe of the pituitary gland where it stops the release of prolactin. As a tumor presses on the stalk, it can interfere with the transport of dopamine to the pituitary gland. When the hypothalamus fails to send signals to the pituitary in the form of dopamine, the pituitary gland produces excessive amounts of the hormone prolactin. Stalk effect may increase prolactin levels to as high as 100 to 150 ng/ml (nanograms per milliliter).
Microprolactinoma tumors are the most common type of pituitary tumor. They are 1 cm in diameter or smaller and elevate prolactin levels to less than 100 to 150 ng/ml. These tumors are most common in premenopausal women.
Macroprolactinoma tumors are larger than 1 cm and typically occur in males. Tumors between 1 cm and 2 cm usually elevate prolactin levels to 200 to 1000 ng/ml. Tumors larger than 2 cm can elevate prolactin levels to more than 1000 ng/ml. Elevated prolactin levels in men seldom produce symptoms but occasionally cause impotence. In rare cases, elevated prolactic levels cause breast swelling with or without milk production.
Some large prolactinomas of the skull base are associated with a falsely low prolactin level due to a laboratory phenomenon known as the "hook effect." This laboratory 'error' may be avoided by requesting that the test for prolactin level be run with dilution (1:100 dilution).
Most prolactin-secreting tumors respond to medical treatment. A trial of bromocriptine (Parlodel) or cabergoline (Dostinex) is usually recommended for all microprolactinomas and for macroprolactinomas associated with prolactin levels higher than 200 ng/ml. If effective, the medication must almost always be taken for life to control the tumor. Rarely, the medication may be stopped without tumor recurrence or without hyperprolactinemia occurring. If the tumor fails to respond to medical management or if the patient is unable to tolerate the medication, surgery is usually considered. Surgery is recommended as the primary treatment for macroadenomas associated with prolactin levels less than 200 ng/ml.
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