ArticlesHistologyBody systemsEndocrine system

ArticlesHistologyBody systemsEndocrine system

Endocrine system

Author: Edwin Ocran, MBChB, MSc • Reviewer: Danijel Tosovic, PhD
Last reviewed: March 27, 2025
Reading time: 7 minutes

The endocrine system consists of specialized glands distributed throughout the body, each producing and releasing hormones directly into the bloodstream. These hormones act as chemical messengers, traveling to specific target organs to regulate essential physiological functions, including growth, metabolism, temperature regulation and reproductive development.

The major components of the endocrine system include the hypothalamus, pituitary gland, pineal gland, thyroid gland, parathyroid glands, thymus, suprarenal glands, pancreas and the gonads (ovaries and testes).

In this article, we will explore the structure of three of the key endocrine glands: the parathyroid, thyroid and suprarenal glands.

Key facts about the endocrine system
Definition	The endocrine system is composed of glands that produce and release hormones into the bloodstream, regulating various physiological functions such as metabolism, growth and stress response.
Parathyroid glands	Location: Four small glands embedded in the posterior surface of the thyroid gland. Hormones produced: Parathyroid hormone (PTH). Function: Controls blood calcium and phosphorus levels by increasing calcium absorption and mobilization.
Thyroid gland	Location: A bilobed gland located in the neck, anterior and inferior to the larynx. Hormones produced: Triiodothyronine (T3), thyroxine (T4) and calcitonin. Function: Regulates metabolism, energy production and calcium homeostasis.
Suprarenal glands	Location: Paired glands situated on the medial aspect of the upper poles of the kidneys. Hormones produced: - Cortex: Mineralocorticoids (aldosterone), glucocorticoids (cortisol), androgens. - Medulla: Epinephrine, norepinephrine, dopamine. Function: The cortex regulates metabolism, immune response and electrolyte balance, while the medulla controls the body's fight-or-flight response by producing stress hormones.

Contents

Parathyroid gland
Thyroid gland
Suprarenal glands
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Parathyroid gland

The parathyroid glands are four small, circular endocrine glands embedded in the posterior surface of the thyroid gland. They secrete parathyroid hormone (PTH), also known as parathormone, which plays a crucial role in regulating calcium and phosphorus levels in the blood.

Each parathyroid gland is enclosed by a thin connective tissue capsule that separates it from the thyroid. Septa extend inward from the capsule, dividing the gland into irregular lobules and providing structural support for the densely packed cords of epithelial cells.

The parathyroid epithelium consists of two main cell types: principal cells, which are responsible for PTH secretion and oxyphil cells, whose function remains less clearly understood.

Take this quiz to test your knowledge about the parathyroid glands:

Thyroid gland

The thyroid gland is a bilobed endocrine organ located in the neck, positioned anterior and inferior to the larynx. Its primary function is to produce, store and secrete the iodine-based hormones triiodothyronine (T3) and thyroxine (T4), which regulate metabolism and other physiological processes.

The gland is enclosed by a thin connective tissue capsule that extends inward, subdividing it into irregular lobular units. Each lobule contains clusters of follicles, which serve as the structural and functional units of the thyroid gland.

Each follicle is surrounded by a thin connective tissue stroma rich in fenestrated capillaries. The follicular epithelium, which lines these structures, varies in morphology based on activity level, ranging from squamous in inactive follicles to cuboidal or low columnar in active ones.

Struggling with histology? Check out these histology slide quizzes to improve your understanding!

In addition to follicular cells, the thyroid also contains parafollicular cells, or C cells. These cells are located within the basal lamina of the thyroid follicles without extending into the follicular lumen, or they may be found in the interfollicular space, either individually or in clusters. C cells play a role in calcium homeostasis by secreting calcitonin, a hormone that lowers blood calcium levels.

Learn more about the thyroid and parathyroid glands by diving into the following study units:

Suprarenal glands

The suprarenal (adrenal) glands are paired retroperitoneal endocrine organs located atop the medial aspects of the upper poles of each kidney. These glands play a crucial role in salt and water balance, metabolism and the body’s fight-or-flight response.

Each suprarenal gland is enclosed by a thick connective tissue capsule, from which trabeculae extend into the gland’s parenchyma. Beneath this capsule lies the cortex of the suprarenal gland, the steroid hormone-secreting region of the gland. The cortex is divided into three distinct zones, each responsible for producing specific hormones:

Zona glomerulosa: Secretes mineralocorticoids, primarily aldosterone, which regulates sodium and potassium balance.

Zona fasciculata: Produces glucocorticoids, such as cortisol, which plays an important role in metabolism and the stress response.

Zona reticularis: Synthesizes adrenal androgens, which serve as precursors for testosterone.

At the core of the suprarenal gland lies the thin, gray medulla, which functions as a neuroendocrine structure and contains chromaffin cells, ganglion cells and dilated capillaries. Chromaffin cells produce the catecholamines: epinephrine (adrenaline), norepinephrine (noradrenaline) and dopamine, which are released directly into the bloodstream to mediate rapid stress responses. While the suprarenal cortex is regulated hormonally, the suprarenal medulla receives direct innervation from preganglionic sympathetic fibers via the splanchnic nerves.

Test your knowledge of the suprarenal glands with this quiz:

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

Moore, K. L., Dalley, A. F., & Agur, A. (2017). Clinically oriented anatomy (8th ed.). Lippincott Williams and Wilkins.
Silverthorn, D. U. (2019). Human Physiology: An Integrated Approach (8th ed.). New York, NY: Pearson Education, Inc.
Standring, S., Anand, N., & Tunstall, R. (2021). Gray's anatomy : the anatomical basis of clinical practice (Forty-second edition). Elsevier

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