The pituitary gland regulates the function of several other endocrine organs.
The pituitary gland consists of two lobes – the anterior and posterior pituitary. They have separate embryological origins and are histologically different, and effectively function as two separate endocrine glands.
Overview of Pituitary Hormones
The table below provides an overview of the hormones secreted from the anterior and posterior pituitary.
|Anterior Pituitary||Posterior Pituitary|
|Growth Hormone (GH)||Antidiuretic Hormone (ADH)|
|Thyroid Stimulating Hormone (TSH)||Oxytocin|
|Follicle Stimulating Hormone (FSH)|
|Luteinising Hormone (LH)|
|Adrenocorticotrophic Hormone (ACTH)|
|Melanocyte stimulating hormone (MSH)|
The anterior pituitary will be focus of the remainder of this article.
Anterior Pituitary Gland
The anterior pituitary releases hormones in response to hormones secreted from the hypothalamus.
The hypothalamus releases hormones into its surrounding interstitial fluid, which permeate into nearby fenestrated capillaries. These capillaries form the hypophyseal portal system that extends into the anterior pituitary. They are histologically similar to veins. However, they connect to each other, rather than to arteries and veins, and provide a rich supply to the pituitary endocrine cells.
The hormones released by the hypothalamus into the anterior pituitary are either releasing hormones (RH) that stimulate the secretion/synthesis of hormones, or inhibiting hormones (IH) that inhibit the synthesis/secretion of hormones.
In response, the anterior pituitary releases hormones into the blood. They have a tropic (“turning on”) effect on another endocrine organ in the body. This means they stimulate another endocrine organ to release a third hormone in the pathway, known as a peripheral hormone. This peripheral hormone travels in the blood stream to exert metabolic actions at different tissues in the body.
Note that in some cases, the hormone secreted by the anterior pituitary hormone may itself be the peripheral hormone in the pathway. Or, it may have dual actions to act directly on tissues, in addition to stimulating a target endocrine gland to release a peripheral hormone.
The peripheral hormone will usually have a positive and/or negative feedback effect on the pituitary and hypothalamus. In other words, it will stimulate or inhibit further release of hypothalamic and/or pituitary hormones. This circuit between the hypothalamus, anterior pituitary and third endocrine gland is known as an axis. A general framework for an axis is depicted below:
There are 5 anterior pituitary axes summarised below:
- Hypothalamic-Pituitary-Adrenal Axis
Involving Adrenocorticotrophic Hormone (ACTH)
- Growth Hormone Axis
Involving Growth Hormone (GH)
- Hypothalamic-Pituitary-Thyroid Axis
Involving Thyroid Stimulating Hormone (TSH)
- Hypothalamic-Pituitary-Gonadal Axis
Involving Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH)
- Prolactin Axis
Involving Prolactin (PRL)
Features of Endocrine Axes
Their activity is maintained within a normal range around a set point, determined by levels of stimulatory and inhibitory hormones in the blood. This means that if the level of the peripheral hormone drops too much, then hypothalamic and pituitary hormones will rise to bring peripheral hormone levels up. Similarly, if the peripheral hormone levels increase, then hypothalamic and pituitary hormone levels will decrease.
Hypothalamic hormones are secreted in a pulsatile manner, governed by internal rhythms. However, they can be regulated by a variety of inputs, including stress, infections and even onset of puberty.
Clinical Relevance – Endocrine Disorders
Endocrine disorders are commonly referred to as being primary, secondary or tertiary diseases. This relates to which organ in the axis is affected.
A primary endocrine disease refers to a disease that affects hormone secretion in the organ that produces the hormone. An example of this would be Addison’s disease, which is where the disease affects the adrenal gland directly and cortisol production. This is sometimes referred to as primary hypoadrenalism.
A secondary endocrine disease affects the endocrine organ that releases tropic hormones, which indirectly affects peripheral hormone secretion. An example here would be Cushing’s disease – a tumour in the pituitary secreting ACTH, resulting in increased adrenal gland activity and raised cortisol production. This would be known as secondary hyperadrenalism.
A tertiary endocrine disease affects the initial endocrine organ in an axis. This is usually a disease of the hypothalamus. Indirectly, it affects a second endocrine organ and then a third endocrine organ, ultimately affecting peripheral hormone levels. Tertiary adrenal insufficiency is a disease of low cortisol caused by a dysfunctional hypothalamus and decreased CRH production.