Pituitary hyperplasia is defined as an absolute increase in the number of one or more cell types of the adenohypophysis. The condition of hyperplasia manifests or is associated with a pituitary enlargement that is detectable using diagnostic imaging. The causes of benign or non-cancerous hyperplasia include hyperstimulation and hypersecretion of neurotransmitters and hormones produced by the hypothalamus and stimulation from hormones produced from peripheral endocrine tissues.
Hyperplasia is further characterized as physiologic, pathologic, and iatrogenic.
Physiologic pituitary hyperplasia results from pregnancy, where prolactin-producing lactotrope cells are stimulated by the increase in progesterone and estrogens to maintain the viability of the implanted embryo.
Pathologic pituitary hyperplasia occurs in response to an end-organ failure. A common example is hypothyroidism, where the response to insufficient circulating thyroid hormone will be an excess production of TSH.
Iatrogenic, or physician-induced pituitary hyperplasia, is caused by the administration of stimulating factors that spur the overgrowth of cell lines in the adenohypophysis. One example is the prolonged use of estrogens in hormone replacement therapy, which may lead to increased prolactin production from an overgrowth of lactotropes.
Pituitary hyperplasia is likely underdiagnosed, with many findings discovered incindentally during brain imaging studies when evaluating head injuries, or during autopsy.
