The endocrine system is a collection of organs that secrete hormones, which are substances that travel through the blood to distant places in the body, where they tell cells what to do. The classical endocrine system includes the pituitary, thyroid, parathyroids, adrenals and pancreas. There are a bunch of other organs – the gut, for example – that also secrete hormones, but they’re not considered part of the classical endocrine system. In this post, we’ll take a big-picture look at how the classical endocrine system works.

Basic organizational structure

The main controlling hub of the system is the pituitary gland, a pea-sized gland at the base of the brain that secretes a bunch of hormones. A few of these hormones travel to distant tissues to give instructions (prolactin, for example, travels to the breast and stimulates milk production). Most pituitary hormones, however, travel to endocrine end-organs and tell them to secrete their own hormones (thyroid stimulating hormone, for example, travels to the thyroid gland and tells it to secrete thyroid hormone).

The pituitary, in turn, is controlled by the hypothalamus, which has its own set of hormones which tell the pituitary what to do (thyroid hormone releasing hormone, for example, tells the pituitary to release thyroid stimulating hormone).

It’s basically a three-tier system: the hypothalamus tells the pituitary what to do, and the pituitary tells most of the other endocrine organs what to do. The cool thing is that the end organ hormones feed back to the hypothalamus, telling it when to turn on or off. If the thyroid hormone level gets too high, for example, the hypothalamus will release less of its thyroid hormone releasing hormone.

With that tiered structure in mind, let’s take a look at the glands of the endocrine system to see what they do and how they are controlled.

Pituitary gland

As mentioned above, this is the controlling hub of the endocrine system. It’s a puny little gland at the base of the brain (weird how such a tiny thing can do so much). It is controlled by the hypothalamus, which releases stimulating and inhibitory hormones that tell the pituitary what to do.

The pituitary has two lobes: an anterior lobe (the “adenohypophysis”) and a posterior lobe (the “neurohypophysis”). In the image above, you can see the anterior lobe on the left and the posterior lob on the right; the hypothalamic stalk is at the top. The anterior lobe is a glandular organ that makes the following hormones:

• Thyroid stimulating hormone (TSH): tells the thyroid to release its hormones
• Adrenocorticotropic hormone (ACTH): tells the adrenals to release their hormones
• Follicle stimulating hormone (FSH): regulates the menstrual cycle
• Luteinizing hormone (LH): regulates the menstrual cycle
• Growth hormone (GH): tells cells to grow, helps regulate blood sugar
• Prolactin (PL): stimulates the mammary glands to produce milk

The posterior lobe is a neural organ (not a glandular organ like the anterior pituitary). It stores and releases a couple hormones that the hypothalamus makes:

• Oxytocin: an incredibly cool hormone that not only helps initiate labor, but is involved in trust, monogamy, and performance on exams (no kidding).
• Vasopressin: helps regulate blood pressure

Thyroid gland

This gland makes thyroid hormone (T4) which is the main controller of the body’s metabolism. It acts all over the body to stimulate things: it makes the skin sweat, the heart pump faster, and the gut move food through faster, for example. Thyroid hormone release is stimulated by TSH.

Parathyroid glands

There are four of these little glands, and they are situated anterior to the thyroid gland. They make parathyroid hormone (PTH), which acts all over the place (bone, kidney, gut) with the ultimate result of raising serum calcium. Secretion of PTH is not controlled by the pituitary, but by calcium levels: when serum calcium goes up, PTH goes down (and vice versa).

Adrenal glands

These are little triangular glands that sit on top of the kidneys. They have a cortex (with three layers) and a medulla. The cortex secretes three main things:

• Mineralocorticoids: help regulate blood pressure
• Glucocorticoids: help regulate blood sugar and a ton of other things, like immune function
• Sex steroids: help regulate reproductive functions, and are important during sexual development

Mineralocorticoid production and secretion are controlled by the renin-angiotensin system (not the pituitary!); glucocorticoid and sex steroid production and secretion are stimulated by ACTH (from the pituitary).

The medulla is a totally different deal. It’s basically a neural organ, and it releases epinephrine and norepinephrine (the fight or flight hormones). It’s not under pituitary control.

Pancreas

The pancreas is kind of cool because it has both exocrine and endocrine glands (which secrete hormones, like insulin). The exocrine part of the pancreas is usually covered together with the digestive system, as it secretes enzymes that help break down food. Exocrine glands, remember, secrete stuff into ducts, not into the blood like endocrine glands. So we’ll set that part aside for now.

The endocrine part of the pancreas consists of little round things called “islets of Langerhans.” The cells in these islets secrete a bunch of hormones, including insulin, which helps get sugar into most cells of the body. The endocrine pancreas is not under pituitary control – it is controlled by other factors in the blood, such as blood sugar. When blood sugar goes up, insulin secretion increases.

Bottom line

In summary, the endocrine system is kind of like a well-run company. The end-organs are little hormone-making factories. The pituitary is the vice president of manufacturing, who tells most of the factories what to do, and the hypothalamus is the CEO, which tells the pituitary how to do its job. Or I suppose you could draw an analogy to academia: president, dean, and department head – but I’d better stop while I’m ahead.