Anterior Pituitary Hormone Deficiencies
There are six types of secretory cells present in the adenohypophysis, including somatotrophs (synthesizing growth hormone), lactotrophs (producing prolactin), mammosomatotrophs (synthesizing both growth hormone and prolactin), thyrotrophs (producing thyrotropin), corticotrophs (synthesizing corticotropin), and gonadotrophs (synthesizing both follicle-stimulating hormone and luteinizing hormone). The synthesis and release of these hormones is well orchestrated under the influence of hypothalamic hormones (most of which are stimulatory and some of which are inhibitory) as well as systemic (endocrine) negative feedback mechanisms, aimed at maintaining homeostatic control.
A wide variety of conditions may cause dysfunction of the hypothalamus or pituitary, leading to selective or universal, partial or complete, acute or chronic loss of adenohypophyseal hormone secretion (anterior hypopituitarism). Any space-occupying lesion impinging on the anterior pituitary, stalk, or hypothalamus may lead to hypopituitarism. In adults, the most common mass lesion in the area of the sella is a benign pituitary adenoma. However, many other neoplasms (including craniopharyngioma, meningioma, chordoma, metastases, or lymphoma), cystic lesions (including Rathke’s cleft cyst or arachnoid cyst), infiltrative (hemochromatosis), inflammatory (hypophysitis, sarcoidosis) or infectious disorders, aneurysm, infarction, primary empty sella, radiation therapy, trauma, surgery, or genetic conditions may all cause hypopituitarism. The underlying cause of hypopituitarism may influence the pattern of hormone loss. Gonadotropin deficiency and growth hormone deficiency tend to occur first in patients with pituitary adenomas or those who have received radiation therapy to the hypothalamus and sella, while thyrotropin and corticotropin function tend to be spared until later in the course of these conditions. In contrast, corticotropin and thyrotropin deficiency frequently occur first in patients with lymphocytic hypophysitis.
Gonadotropin deficiency presents as lack of pubertal development in adolescents, who generally develop a eunuchoid habitus. If the onset of gonadotropin deficiency occurs in adulthood, patients present with loss of gonadal function, including oligomenorrhea or amenorrhea in women, and erectile dysfunction in men. In addition, low libido and infertility may occur in patients of both genders. Patients may also experience loss of body hair (particularly in the presence of concurrent corticotropin deficiency), fine facial wrinkling, loss of bone calcium leading to increased fracture risk, and hot flashes. Women may also experience breast atrophy, vaginal dryness, and dyspareunia. Men may note loss of stamina, increased body fat, decreased lean body mass, and decreased testicular size. Prolactin deficiency may result in failure of lactation postpartum.
Growth hormone deficiency leads to decreased linear growth if it occurs in childhood or adolescence. In adulthood, loss of growth hormone secretion is more subtle, but may be associated with fatigue, decreased exercise capacity and muscle strength, abnormal body composition (decreased lean body mass, loss of bone calcium, and gain in body fat), dyslipidemia, insulin resistance, increased cardiovascular risk, and poor quality of life.
Thyrotropin deficiency leads to central hypothyroidism, including fatigue, lethargy, weight gain, bradycardia, dry skin, myxedema, anemia, constipation, muscle aches, decreased relaxation phase of Achilles reflexes, and cold intolerance.
Corticotropin deficiency leads to central hypoadrenalism, which is potentially the most life threatening of all pituitary hormone deficiencies. These patients often exhibit fatigue, weight loss, nausea and vomiting, orthostatic hypotension and dizziness, and diffuse arthralgias. Notable is the lack of cutaneous and mucosal hyperpigmentation, in contrast to patients with primary adrenal insufficiency (Addison disease). These patients may also present acutely with shock unresponsive to volume expansion and pressors. Eosinophilia and hyponatremia may be present. However, hyperkalemia is absent, because aldosterone deficiency does not occur.
Once diagnosed, target organ hormone replacement therapies are instituted. In particular, glucocorticoid replacement may prove lifesaving in patients presenting in adrenal crisis. Levothyroxine is used to replace central hypothyroidism, and sex steroid replacement is used to replace patients with central hypogonadism. However, if fertility is of interest, gonadotropin therapy is used, including human chorionic gonadotropin and follicle-stimulating hormone. Growth hormone replacement may also be considered. Despite seemingly adequate replacement therapies, patients with hypopituitarism are at increased risk of cardiovascular mortality, the underlyi g reasons still being a matter of considerable debate.