The pituitary gland, often recognized as the “master gland,” is a small, pea-sized structure located at the base of the brain. Its primary function is to produce and release hormones that regulate other endocrine glands throughout the body, including the thyroid, adrenal glands, and gonads. Testing for pituitary function aims to determine if the gland is producing an appropriate amount of hormones, which can manifest as either too little (hypofunction or hypopituitarism) or too much (hyperfunction). The diagnostic process is comprehensive, moving from simple baseline measurements to complex provocative tests and structural imaging, all necessary to pinpoint the exact nature of any dysfunction.
Static Hormone Measurement
The initial step in evaluating pituitary function involves static hormone measurement, which is typically a standard blood test to check baseline hormone levels at a single point in time. Hormones secreted by the anterior pituitary are measured, such as Prolactin, Thyroid-Stimulating Hormone (TSH), Follicle-Stimulating Hormone (FSH), and Luteinizing Hormone (LH). Growth Hormone (GH) and a morning sample of Adrenocorticotropic Hormone (ACTH) and cortisol may also be included.
These direct measurements provide only a snapshot of the body’s complex endocrine environment. Because the pituitary controls other glands, a complete assessment requires measuring the hormones produced by those target glands as well. For example, TSH levels must be evaluated alongside the thyroid hormone Free T4 to correctly assess the feedback loop between the pituitary and the thyroid. A low TSH combined with a low Free T4 suggests a problem at the pituitary level, known as secondary hypothyroidism.
A single static measurement can sometimes be insufficient for a definitive diagnosis because some hormones, like GH and ACTH, are released in pulsatile bursts throughout the day. This variable secretion means a normal result at one moment might not reflect a deficiency or excess that occurs at other times. Therefore, if initial static tests are inconclusive, further, more dynamic testing is required.
Dynamic Stimulation and Suppression Tests
When static hormone levels are inconclusive or suggest a specific disorder, dynamic testing is used to provoke or suppress the pituitary gland’s activity. These tests involve administering a specific substance—a drug, hormone, or glucose—and then taking timed blood draws to see how the pituitary or its target gland responds. This helps determine if the gland has a sufficient hormone reserve (stimulation) or if its hormone production can be appropriately shut off (suppression).
Stimulation tests are frequently used to confirm hypofunction or hormone deficiency. For example, the short Synacthen test involves injecting a synthetic form of ACTH to see if the adrenal glands produce a sufficient amount of cortisol, indirectly testing the pituitary-adrenal axis. The Insulin Tolerance Test (ITT) is considered a gold standard for assessing the reserve of both ACTH and GH, as induced low blood sugar is a powerful stressor that should stimulate the release of both hormones. A failure to achieve an adequate rise in cortisol or GH during this test confirms a lack of reserve.
Conversely, suppression tests are used to confirm hormone overproduction or hyperfunction. The Oral Glucose Tolerance Test (OGTT) is a common suppression test for suspected Growth Hormone excess, a condition known as acromegaly. In healthy individuals, consuming glucose should suppress GH levels, but in patients with acromegaly, the GH level fails to suppress. For suspected excess cortisol production (Cushing’s syndrome), the Dexamethasone Suppression Test is performed, where the synthetic steroid dexamethasone is given to see if it can suppress the pituitary’s ACTH production and, consequently, the adrenal gland’s cortisol release.
Imaging Studies for Structural Integrity
Beyond functional testing, visualizing the pituitary gland is necessary to identify any structural issues that might be causing the hormone imbalance. Magnetic Resonance Imaging (MRI) is the preferred method for assessing the pituitary gland and the surrounding sellar region. MRI provides superior soft tissue contrast compared to a Computed Tomography (CT) scan, allowing for a detailed depiction of the gland’s anatomy.
Dedicated MRI protocols are used, often including both non-contrast and contrast-enhanced sequences. This imaging is used to look for structural problems such as adenomas, which are benign tumors and the most common cause of pituitary dysfunction. MRI can detect microadenomas (tumors less than 10 millimeters) as well as larger macroadenomas, cysts, or inflammation of the gland.
Understanding Test Results and Next Steps
Interpreting pituitary function tests requires a careful correlation of all the information gathered from the static measurements, dynamic tests, and imaging studies. For instance, consistently high levels of Prolactin combined with the failure of suppression in dynamic tests, and the presence of a tumor on MRI, would strongly indicate a prolactinoma. Conversely, a low morning cortisol that does not increase adequately during a stimulation test, alongside a normal-sized gland on MRI, suggests a functional deficiency.
High hormone levels generally indicate hyperfunction, often due to an over-secreting tumor, while low levels suggest hypofunction, which can result from damage to the pituitary or its regulating hypothalamus. A definitive diagnosis rests on the combination of a clinical picture, biochemical confirmation of a hormone excess or deficiency, and the identification of a structural cause through imaging. Once the diagnosis is established, a specialist in endocrine disorders, known as an endocrinologist, determines the appropriate course of action. Treatment options can range from medication to replace deficient hormones or block excess ones, to surgery or radiation therapy, and often require long-term monitoring.