The pituitary gland produces and secretes various hormones that play a vital role in regulating endocrine function within the body. The pituitary gland consists of 2 lobes: an anterior and a posterior lobe. Hormones produced by the anterior lobe of the pituitary gland include growth hormone (GH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH), follicular stimulating hormone (FSH), adrenocorticotropin (ACTH), and prolactin. Hormones stored and released from the posterior pituitary are antidiuretic hormone (ADH) or vasopressin and oxytocin. ADH and oxytocin are produced by neurosecretory cells in the hypothalamus. Trophic hormones produced by the hypothalamus stimulate the production of different anterior pituitary hormones which in turn stimulate the production of hormones at the level of the target organ. Negative feedback by hormones produced by the target organ and tissue inhibit further production of the related pituitary hormones. Readers are directed to the Statpearls article on hypopituitarism for further details regarding pituitary hormone regulation.
Hyperpituitarism is defined as an excessive secretion or production of 1 or more of the hormones produced by the pituitary gland. In this article, we present an overview of the diagnosis and management of secretory tumors of the pituitary gland.
Prolactin-secreting pituitary tumors are known as prolactinomas. They are the most frequent secretory tumor of the pituitary gland accounting for at least 40% of all pituitary tumors. These are reported more commonly in women. Other causes of increased prolactin, apart from pituitary adenomas, include craniopharyngioma and other sellar or parasellar masses, infiltration of the hypothalamus, and head trauma. Mild elevations in prolactin may also be seen in patients with hypothyroidism, polycystic ovarian disease, and secondary to use of certain medications such as antipsychotic agents, opioid analgesics, antidepressants, among others. Physiological causes of increased prolactin levels include stimulation of the nipple and sexual intercourse.
Excessive production of GH leads to the clinical conditions acromegaly and gigantism. The incidence of acromegaly is 5 cases per million per year, and the prevalence is 60 cases per million. Over 95% of patients with acromegaly have a GH secreting pituitary adenoma derived from the somatotroph cell line. In less than 5% of cases, acromegaly is a result of excessive GH releasing hormone (GHRH) secretion from a hypothalamic or a neuroendocrine tumor. Ectopic production of GH is extremely rare.
Excessive ACTH production leads to the presence of hypercortisolism (Cushing syndrome). The incidence of Cushing syndrome is 2 to 3 cases per million. Pituitary adenomas (Cushing disease) accounts for greater than 80% of ACTH dependent hypercortisolism, and 10% to 20% are secondary to the ectopic production of ACTH. Cushing disease is more common in females than in males with a ratio of 3:1.4.
Pituitary adenomas that produce TSH (TSHoma) represent less than 1% of functioning pituitary adenomas, with the Swedish National Registry reporting an incidence rate of 0.15 cases per million inhabitants per year. Co-secretion of GH and prolactin is common.
The presence of a pituitary adenoma with pancreatic endocrine tumors and the parathyroid tumor is associated with multiple endocrine neoplasia (MEN) type 1 syndromes.
The incidence of hyperpituitarism is usually reported with a specific condition associated with the particular pituitary hormone excess. The incidence of each related pituitary hormone excess is indicated under the etiology section of this article.
Presentation of patients with hyperpituitarism will depend on the hormone or hormones that are produced in excess, whether there is a mass pituitary lesion present, and whether there are associated features of hypopituitarism. Local effects of a pituitary tumor such as visual field disturbances and headache may be present in the presence of a mass lesion. Pituitary tumors can be microadenomas less than 10 mm or macroadenomas greater than 10 mm. Macroadenomas usually cause hypopituitarism.
Excessive production of prolactin may occur in isolation or can occur with increased production of other hormones such as GH. Clinical features include infertility and galactorrhoea and secondary amenorrhea in females, loss of libido, and impotence in males. In males, the tumors are usually macroadenomas presenting with features of extension beyond the pituitary.
Excessive Production of Growth Hormone
Excessive production of GH in adults results in the condition known as acromegaly. Features include:
In children, prior to fusion of epiphyseal plates in long bones (for example femur and tibia), excessive GH production results in gigantism.
Excessive production of TSH causes secondary hyperthyroidism. Clinical features are those of thyroid hormone excess and include weight loss, heat intolerance, anxiety, menstrual disturbances, and palpitations. A diffuse goiter may also be present. Hyperthyroid symptoms with TSHomas are often mild to moderate.
Excessive production of ACTH by the pituitary results in Cushing disease. Clinical features include central obesity, skeletal muscle wasting, the presence of buffalo hump, striae, excessive bruising, menstrual abnormalities, increased blood pressure, glucose intolerance, as well as depression and psychosis.
The initial tests should be directed at the suspected excess hormone. Additionally, the possible deficiencies of other pituitary hormones should also be considered and relevant testing performed.
Basal levels of prolactin are useful with values of greater than 200 ng/mL associated with the presence of a prolactinoma. Prolactin levels generally are in parallel to the tumor size. Prolactin (PRL) may be increased due to other causes not related to pituitary disease as described above. MRI and basal PRL levels are sufficient for the diagnosis. Provocative tests are not indicated. If PRL levels are normal with clinical features then it is possible that there is assay interference, the “hook effect” and samples should be diluted to provide a reliable result since the excess antigen, PRL is causing a prozone effect. Rarely patients have an increase in PRL due to macroprolactinemia which is a harmless condition and can be diagnosed by contacting the laboratory for precipitating the big PRL.
Random GH levels are usually not recommended due to the diurnal and pulsatile nature of secretion that occurs during a day. Dynamic function (suppression testing) forms the cornerstone of laboratory investigations of GH excess. The oral glucose suppression test involves intake of 75 gm of glucose with measurement of GH levels at zero, 60, and 120 minutes. A GH level of less than 1 ug/L usually excludes the diagnosis of acromegaly. The presence of increased IGF1 levels accompanies excessive GH production. IGF1 measurement does not require the use of fasting sample or suppression testing as a random sample adequately reflects the influence of GH excess/deficiency. Thus it is recommended as the initial screening tests for acromegaly. It is vital that the relevant age and sex-matched reference intervals be used when interpreting IGF1 results. As the liver under the action of GH produces IGF1, significant liver disease may result in decreased IGF1 production. Other conditions affecting IGF1 results include renal failure, hypothyroidism, poorly controlled diabetes mellitus and severe malnutrition. Around 30% of patients with Acromegaly can have increased prolactin levels.
ACTH (Cushing Disease)
The laboratory investigation of Cushing syndrome includes confirmation of excessive corticosteroid production followed by identification of ACTH dependent/independent production and determination of the site of ACTH production (whether ectopic production is present).
Tests for Confirmation of Hypercortisolism
Determination of ACTH Dependence/Independence
The CRH stimulation test may be accompanied by bilateral inferior petrosal sinus sampling (BIPSS) to confirm the presence of pituitary lesion causing Cushing syndrome. During this invasive procedure, the inferior petrosal sinuses into which the pituitary venous blood drains is catheterized by a radiologist. ACTH is measured at baseline and following stimulation with 100 micrograms of CRH from both IPS and peripheral veins. Measurement of prolactin is also performed to confirm catheter is in the correct position. A ratio of IPS to peripheral ACTH of 2:1 before CRH stimulation and 3:1 after stimulation indicates the pituitary cause of ACTH production consistent with Cushing's Disease has also been used to assist in lateralization of the pituitary lesion.
Secondary hyperthyroidism presents with increased or unsuppressed TSH levels and elevated thyroid hormone (free and total T4 and T3) levels. This picture is also seen in the presence of laboratory interference and thyroid hormone resistance both of which are more commonly occurring than TSH producing pituitary tumors. Since glycoprotein hormones have alpha and beta subunits, tumors can produce an excess of alpha subunit in TSHoma.
Thyroid releasing hormone (TRH) if available, stimulation test can be used to verify if the TSH is of pituitary origin. Following administration of TRH, TSH is measured. Patients with a TSH producing tumor have an impaired response.
FSH, LH, estradiol, and testosterone levels may be useful in ascertaining deficiency of these hormones secondary to a pituitary tumor. Rarely elevated FSH and LH may be associated with a gonadotropin cell adenoma.
Pituitary imaging studies using unenhanced or gadolinium-enhanced MRI are preferred to CT scans for visualization of pituitary adenomas. Often pituitary adenomas are discovered incidentally when imaging studies have been done for other reasons.
Management of hyperpituitarism will depend on the cause and the hormone or hormones affected.
Pharmacological treatment includes the use of somatostatin analogs and competitive receptor antagonists. For prolactinomas, medical therapy is the usual choice and consists of dopamine agonists such as bromocriptine and cabergoline.
Resection of pituitary and tumors is technically challenging because of the anatomical location. Minimally invasive procedures are increasingly utilized. These include trans-sphenoidal surgery for acromegaly, prolactinoma-macroadenoma and Cushing syndrome with an adenoma. However, as many patients with pituitary tumors often present late with large tumors complete resection is often not possible.
Conventional radiotherapy may be used to reduce tumor size however pituitary damage and resultant hypopituitarism may result.