Hypopituitarism is a health condition characterized by the reduced production of hormones from the pituitary gland. The pathophysiology of hypopituitarism usually involves damage to the pituitary gland, which renders it unable to produce one or more hormones in the normal manner.
The deficiency of these tropic hormones leads to secondary functional deficits of the target endocrine glands, leading to symptoms caused by the insufficient hormone production.
Physiology of the Pituitary Gland
The pituitary gland consists of two parts: the anterior and the posterior pituitary. The anterior pituitary receives stimulatory or inhibitory hormonal signals from the hypothalamus. It in turn produces other stimulating or tropic hormones to be secreted into the systemic circulation.
These then exert their effects on various target tissues, such as the adrenal cortex and medulla, the thyroid, the parathyroids and the gonads. It is important to note that most of these hormones do not directly produce the final physiologic effect required by the body. They rather stimulate the organs responsible for the final function, and modulate body activity to suit the circumstances under which it acts. The exceptions are the growth hormone and prolactin.
The posterior pituitary does not produce hormones. It stores and secretes hormones produced in the hypothalamus and transmitted to it via the portal vessels that link the two glands.
The pituitary hormones that are produced by the anterior pituitary gland include:
- Adrenocorticotropic hormone (ACTH)
- Thyroid stimulating hormone (TSH)
- Luteinizing hormone (LH) and follicle-stimulating hormone (FSH)
- Growth hormone (GH)
The pituitary hormones that are secreted by the posterior pituitary gland include:
- Antidiuretic hormone (ADH), also known as vasopressin (VP)
- Oxytocin (OXT)
A deficiency in one or more of the anterior or posterior pituitary hormones is referred to as hypopituitarism and can have various effects throughout the body.
Pathophysiology of Hypopituitarism
When the production of one or more pituitary hormones is impaired, the gland targeted by the tropic pituitary hormone will have a reduced secretory action, in its turn. Under normal circumstances, a reduction in the action of the target gland, or in other words, a reduced concentration of the secondary hormones, stimulates the pituitary gland to produce more of the hormone to strengthen the effect. This is called a feedback loop.
However, in patients with hypopituitarism, the pituitary gland or some of its component secretory cells are unable to respond adequately to a reduction in the target hormone levels. As a result, the action of the target glands continues to remain low. This eventually leads to a deficiency in the hormones produced by the target glands, and a reduction in their physiologic function. The end result is a failure of some body process.
Unlike the assessment of the target glands, the function of the pituitary gland is usually assessed by the function of the target gland in combination with the concentration of the stimulating or tropic pituitary hormone. For example, the function of the pituitary gland in regulating thyroid function is assessed by the concentration of the secondary hormone, thyroxin, in the blood as well as that of the tropic hormone thyrotropin or thyroid stimulating hormone.
A low level of thyroxin with an inappropriately low or even normal thyrotropin level would, therefore, indicate pituitary disease and not thyroid disease. This is because the tropic hormone would be increased in response to the low thyroid hormone concentration in blood, by the normal pituitary.
There are many possible causes of hypopituitarism. The most common is a pituitary adenoma or other tumor of the pituitary tissue. Other causes of trauma to the head and brain may also cause damage to the pituitary gland and affect its ability to produce hormones. These include:
- traumatic brain injury (TBI)
- subarachnoid hemorrhage
- Sheehan syndrome
Damage to the pituitary gland due to a tumor may occur either because of:
- impaired blood flow to the gland
- compression of the pituitary tissue caused by the tumor growth, or by related inflammation.
Many other circumstances may cause damage to the pituitary gland in a similar manner and cause hypopituitarism. These include brain surgery, radiation treatment and inflammatory conditions or infiltrative diseases.