How The Pituitary Works

The pituitary gland, situated at the base of the brain (behind the top of the nose) and below the hypothalamus, regulates various hormonal axes, including those leading to the production of growth hormone, reproductive hormones, thyroid hormone, and stress hormone.  

The pituitary hormones are part of hypothalamus-pituitary-end organ axes. The pituitary regulates these pathways, sensing if levels of end-organ hormones are too low or too high, and altering secretion of its hormones accordingly.

The anterior pituitary produces growth hormone (GH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), and prolactin. With the exception of prolactin, anterior pituitary hormone secretion is stimulated by hormones from the hypothalamus. Secretion of the hormones from the pituitary and hypothalamus are also stimulated by a fall in production of end-organ hormone.

In a functioning system, the body regulates itself to maintain equilibrium.

For example, thyrotropin-releasing hormone secreted from the hypothalamus signals the pituitary to produce thyrotropin, or thyroid stimulating hormone (TSH). TSH signals the thyroid gland to secrete thyroid hormone (shown as T3 and T4 in the figure below). If insufficient thyroid hormone is produced, a feedback loop signals both the hypothalamus and the pituitary to increase the stimulatory signal— that is, TSH increases. Conversely, too much thyroid hormone (whether from an overactive gland or exogenous thyroid hormone) suppresses TSH secretion via negative feedback to the pituitary (and hypothalamus).

Understanding these feedback loops allows proper interpretation of test results. TSH alone is commonly used as a screening test for thyroid dysfunction; this is appropriate if the source of hypothyroidism is the thyroid gland itself, as low thyroid levels will lead to an abnormally high TSH. If the pituitary or hypothalamus are damaged, the feedback system does not function appropriately, and a lone TSH will miss a diagnosis of hypothyroidism: an injured pituitary cannot respond sufficiently to low thyroid hormone levels, and TSH will be inappropriately normal. In sum, “normal” can be misleading.

Diurnal variation is another important feature of neuroendocrine hormones. Evaluation for a low hormone level is best conducted during the expected peak time, to reduce the risk that a low result represents a normal relative nadir. Both cortisol and testosterone peak in the morning (with normal diurnal schedules), and evaluation for deficiencies should thus occur in the morning. As any one blood draw represents a single time point, it may be necessary to proceed to provocative testing or to conduct serial tests.