Endocrine Organs and Hormones

Endocrine glands are distributed throughout the body — some in the brain (hypothalamus, pituitary, pineal gland), some in the neck and torso (thyroid, parathyroid glands, adrenal glands, pancreas), and some as paired reproductive organs (the testes and ovaries). This page walks through each major endocrine organ, the hormones it secretes, their target tissues and effects, and the feedback loops that keep them in balance.

Key Takeaways

• The hypothalamus controls the anterior pituitary via tropic hormones (GnRH, GHRH, TRH, CRF) — except prolactin, which is inhibited by dopamine rather than stimulated.

• The anterior pituitary produces seven hormones (FLAT PEG): four tropic (FSH, LH, ACTH, TSH) and three direct (prolactin, endorphins, growth hormone).

• The posterior pituitary stores and releases oxytocin and ADH, both actually made in the hypothalamus.

• The thyroid produces T₃/T₄ (raise metabolic rate) and calcitonin (lowers blood calcium); the parathyroid glands produce PTH (raises blood calcium) — calcitonin and PTH are direct opposites.

• The adrenal cortex produces glucocorticoids (cortisol, ACTH-regulated), mineralocorticoids (aldosterone, RAAS-regulated), and cortical sex hormone precursors; the adrenal medulla produces catecholamines for the fight-or-flight response.

• The pancreas balances blood glucose via glucagon (α-cells, raises glucose), insulin (β-cells, lowers glucose), and somatostatin (δ-cells, inhibits both) — imbalance underlies Type 1 and Type 2 diabetes mellitus.

• The gonads (testosterone; estrogen/progesterone) and pineal gland (melatonin) round out the classic endocrine organs, while the GI tract, kidneys, heart, and thymus also secrete important hormones.

The Hypothalamus: Endocrine Control Center

The hypothalamus links the nervous system to the endocrine system by regulating the pituitary gland through tropic hormones. Located in the forebrain just above the pituitary, it communicates directly with the anterior pituitary through the hypophyseal portal system — a specialized blood vessel network that delivers hypothalamic hormones to the anterior pituitary quickly and in high concentrations.

Hypothalamic regulation runs on negative feedback: when a downstream hormone's levels get too high, it inhibits further hypothalamic release, keeping the system in balance.

The hypothalamus also has functionally distinct regions: the anterior hypothalamus is involved in sexual behavior, the ventromedial hypothalamus signals satiety (fullness), and the lateral hypothalamus acts as the hunger center, triggering eating behavior.

Hypothalamus → anterior pituitary hormone pairings:

• GnRH (gonadotropin-releasing hormone) → stimulates FSH and LH release, which act on the gonads.

• GHRH (growth hormone-releasing hormone) → stimulates growth hormone release.

• TRH (thyroid-releasing hormone) → stimulates TSH release, which acts on the thyroid.

• CRF (corticotropin-releasing factor) → stimulates ACTH release, which acts on the adrenal cortex.

• Exception — prolactin: instead of a releasing hormone, prolactin is inhibited by dopamine (also called prolactin-inhibiting factor) from the hypothalamus. As long as dopamine is released, prolactin secretion is suppressed; when dopamine drops, prolactin is secreted.

Negative feedback governs this whole system: for example, high blood cortisol inhibits further release of CRF from the hypothalamus and ACTH from the anterior pituitary, preventing overshoot.

The Pituitary Gland: The "Master Gland"

The pituitary gland is divided into two distinct lobes — the anterior pituitary and the posterior pituitary — each releasing its own set of hormones.

Anterior Pituitary (FLAT PEG)

The anterior pituitary is a true endocrine organ: it both synthesizes and secretes hormones, producing seven in total. Four are tropic (stimulate other endocrine glands); three are direct (act on non-endocrine target tissues). The mnemonic FLAT PEG covers all seven:

Growth hormone dysregulation, by timing:

• Excess before growth plates close (childhood) → gigantism (unusually tall stature).

• Deficiency during childhood → dwarfism (short stature from underdeveloped long bones).

• Excess after growth plates close (adulthood) → acromegaly (bones don't grow longer, but soft tissues and certain bones — jaw, hands, feet — thicken).

Posterior Pituitary (Oxytocin and ADH)

The posterior pituitary doesn't produce its own hormones. Instead, it stores and releases two hormones that are actually synthesized by neurons in the hypothalamus, transported down their axons, and stored in the posterior pituitary until needed:

• Oxytocin: stimulates uterine contractions during labor and promotes milk ejection during breastfeeding. It operates through a positive feedback loop — a stimulus like a baby suckling leads to more hormone release.

• Antidiuretic hormone (ADH / vasopressin): increases water reabsorption in the kidneys, released when blood is too concentrated. It acts on the kidney's collecting ducts, making them more permeable to water — more water is reabsorbed, urine becomes more concentrated, and the body conserves water.

The Thyroid Gland

The thyroid is a butterfly-shaped gland at the front of the neck that regulates metabolism and helps maintain calcium homeostasis. It's controlled by TSH from the anterior pituitary, which is itself controlled by TRH from the hypothalamus — a three-tier hormone cascade.

T3 and T4

T₃ and T₄ are synthesized by the thyroid's follicular cells by combining iodine with the amino acid tyrosine. They act on nearly every cell in the body to raise the metabolic rate, by increasing cellular respiration and increasing glucose/fatty acid utilization. A negative feedback loop keeps them in check: high T₃/T₄ levels inhibit both TRH (hypothalamus) and TSH (anterior pituitary).

Hypothyroidism vs. hyperthyroidism:

• Hypothyroidism (too little T₃/T₄): cold intolerance, weight gain, fatigue. In children, severe cases can cause cretinism — delayed growth and mental development.

• Hyperthyroidism (too much T₃/T₄): heat intolerance, weight loss, restlessness, anxiety — driven by an overactive metabolic rate.

Calcitonin

Calcitonin is secreted by parafollicular cells (C-cells) in the thyroid and lowers blood calcium through three mechanisms: increasing calcium excretion by the kidneys, decreasing calcium absorption in the gut, and increasing calcium storage in bone.

The Parathyroid Glands

The parathyroid glands are four small glands on the posterior surface of the thyroid. They secrete parathyroid hormone (PTH) in response to low blood calcium, raising calcium levels back to normal through three mechanisms: stimulating bone resorption (releasing calcium from bone into the blood), increasing calcium reabsorption in the kidneys, and activating vitamin D, which increases intestinal calcium absorption. PTH also promotes phosphate excretion in the urine, maintaining a healthy calcium-to-phosphate ratio. Like most hormonal systems, PTH release is suppressed by negative feedback when blood calcium rises above normal.

Calcitonin vs. parathyroid hormone: these two hormones work in direct opposition. Calcitonin (thyroid) lowers blood calcium; parathyroid hormone (parathyroid glands) raises it. Together, they keep blood calcium balanced.

The Adrenal Glands

The adrenal glands sit atop each kidney and are divided into two parts: the outer adrenal cortex and the inner adrenal medulla, each releasing different hormones.

Adrenal Cortex (Glucocorticoids, Mineralocorticoids, Cortical Sex Hormones)

The adrenal cortex produces three classes of steroid hormones:

• Glucocorticoids (cortisol, cortisone): involved in the stress response — increase blood glucose, reduce protein synthesis, and suppress the immune system (reducing inflammation). Release is triggered by ACTH from the anterior pituitary, which is itself regulated by CRF from the hypothalamus.

• Mineralocorticoids — chiefly aldosterone: acts on the distal convoluted tubule and collecting duct of the kidney nephron, promoting sodium reabsorption (water follows, increasing blood volume and blood pressure) along with potassium and hydrogen ion excretion. Unlike glucocorticoids, aldosterone isn't primarily regulated by ACTH — it's controlled by the renin-angiotensin-aldosterone system (RAAS).

• Cortical sex hormones: the adrenal cortex primarily secretes weak androgen precursors — chiefly DHEA (dehydroepiandrosterone) and androstenedione — which peripheral tissues convert into testosterone and estrogens. Although the gonads remain the primary source of sex hormones, this adrenal contribution matters more in females, where it plays a larger relative role in secondary sex characteristics, libido, and reproductive function.

Adrenal Medulla (Catecholamines)

The adrenal medulla differs from the cortex in both origin and function — it develops from neural crest tissue (the cortex develops from mesoderm) and functions like a modified extension of the sympathetic nervous system, releasing hormones directly into the blood rather than sending nerve impulses.

These hormones are catecholamines: epinephrine (adrenaline) and norepinephrine (noradrenaline), central to the fight-or-flight response. They promote glycogen breakdown into glucose in the liver (raising blood sugar), increase the basal metabolic rate, increase heart rate, dilate the bronchi, and redirect blood flow — vasoconstriction in non-essential areas like the digestive tract, vasodilation in skeletal muscle and heart tissue — so oxygen and nutrients go where they're needed most.

The Pancreas

Alongside its digestive role, the pancreas maintains glucose homeostasis through the islets of Langerhans — clusters containing three hormone-secreting cell types: α-cells, β-cells, and δ-cells.

Glucagon, Insulin, and Somatostatin

• Glucagon (α-cells) raises blood glucose when it drops too low — between meals or during fasting — via glycogenolysis (breaking down stored glycogen into glucose), gluconeogenesis (generating new glucose from non-carbohydrate sources like amino acids and fats), and lipolysis/protein catabolism.

• Insulin (β-cells) lowers blood glucose after eating by stimulating glucose uptake (especially in muscle and fat tissue) and promoting anabolic processes: glycogenesis (glucose storage as glycogen), lipogenesis (energy storage as fat), and protein synthesis.

• Somatostatin (δ-cells) acts like a brake, inhibiting both insulin and glucagon secretion to prevent excessive swings in blood glucose.

Insulin and glucagon work like a seesaw: glucagon raises blood sugar, insulin lowers it.

Diabetes Mellitus

When the insulin-glucagon balance breaks down, the result is chronic hyperglycemia — diabetes mellitus — either because the body doesn't make enough insulin, or because cells become insensitive to it.

Both types cause hyperglycemia (high blood sugar). The kidneys try to flush out excess glucose through urine, causing polyuria (frequent urination), which in turn causes polydipsia (excessive thirst) as the body dehydrates. The opposite problem — blood glucose dropping too low, from excess insulin or missed meals — is hypoglycemia, causing shakiness, confusion, and fatigue.

The Gonads

The gonads serve a dual role as reproductive organs and endocrine glands. In males, the testes produce testosterone, driving development of the male reproductive system and secondary sex characteristics like a deepening voice, facial hair, and increased muscle mass. In females, the ovaries secrete estrogen and progesterone, regulating the menstrual cycle, maintaining pregnancy, and contributing to female secondary sex characteristics like breast development and hip widening.

The Pineal Gland

The pineal gland is a small, pinecone-shaped structure deep within the brain. It releases melatonin, a hormone that regulates circadian rhythms — the body's natural sleep-wake cycle. Melatonin production is influenced by light exposure.

Other Hormone-Secreting Organs

Several organs release hormones without being classified as traditional "endocrine glands":

• Stomach and intestines: gastrin stimulates gastric acid release to aid digestion; secretin regulates small intestine pH by stimulating pancreatic bicarbonate release; cholecystokinin (CCK) stimulates gallbladder bile release and pancreatic enzyme secretion.

• Kidneys: erythropoietin (EPO) stimulates the bone marrow to produce more red blood cells, especially when blood oxygen levels are low.

• Heart: the atria release atrial natriuretic peptide (ANP) when blood volume or pressure rises and stretches the atria — ANP promotes salt and water loss through urine, reducing blood volume and pressure.

• Thymus: most active during childhood, it secretes thymosin, essential for the development and maturation of T-cells (a type of white blood cell in adaptive immunity).

Common MCAT Mistakes

• Mixing up calcitonin and parathyroid hormone. Calcitonin (thyroid C-cells) lowers blood calcium; PTH (parathyroid glands) raises it. They're direct opposites, not two names for the same effect.

• Assuming aldosterone is regulated by ACTH like cortisol. Glucocorticoids (cortisol) are ACTH-driven. Aldosterone, a mineralocorticoid, is controlled separately by the renin-angiotensin-aldosterone system (RAAS), triggered by low blood pressure/volume.

• Confusing the adrenal cortex and adrenal medulla. The cortex (mesoderm-derived) makes steroid hormones — glucocorticoids, mineralocorticoids, sex hormone precursors. The medulla (neural crest-derived) makes catecholamines and acts like an extension of the sympathetic nervous system.

• Treating prolactin like the other anterior pituitary hormones. Most anterior pituitary hormones are turned on by a hypothalamic releasing hormone. Prolactin is the exception — it's normally suppressed by dopamine, and secretion rises when dopamine drops.

MCAT-Style Concept Check

Question: A patient has chronically low blood pressure, and lab work shows elevated renin and elevated aldosterone. Which endocrine organ is directly responsible for releasing the hormone that raises this patient's blood pressure, and by what kidney mechanism?

• A) Adrenal medulla, via epinephrine acting on the heart

• B) Adrenal cortex, via aldosterone acting on the distal convoluted tubule and collecting duct

• C) Posterior pituitary, via ADH acting on the collecting duct

• D) Kidney, via renin acting directly on blood vessels

Answer: B

Explanation: Low blood pressure triggers renin release from the kidneys, which converts angiotensinogen to angiotensin I; ACE (mainly in the lungs) then converts this to angiotensin II, which stimulates the adrenal cortex to release aldosterone. Aldosterone acts on the distal convoluted tubule and collecting duct of the nephron, promoting sodium (and water) reabsorption, which raises blood volume and blood pressure. The adrenal medulla, by contrast, releases catecholamines and isn't part of the RAAS pathway.

FAQ

What's the difference between the anterior and posterior pituitary?

The anterior pituitary synthesizes and secretes its own seven hormones (FLAT PEG), most under hypothalamic tropic control. The posterior pituitary doesn't make its own hormones — it stores and releases oxytocin and ADH, which are actually synthesized by hypothalamic neurons.

Why does prolactin work differently from the other anterior pituitary hormones?

Every other anterior pituitary hormone is stimulated by a hypothalamic releasing hormone (GnRH, GHRH, TRH, CRF). Prolactin is the exception — it's suppressed by dopamine (prolactin-inhibiting factor); when dopamine drops, prolactin secretion rises.

What's the difference between the adrenal cortex and adrenal medulla?

The adrenal cortex is the outer layer, develops from mesoderm, and produces steroid hormones (glucocorticoids, mineralocorticoids, cortical sex hormones). The adrenal medulla is the inner layer, develops from neural crest tissue, and produces catecholamines (epinephrine, norepinephrine) as part of the fight-or-flight response.

How do insulin and glucagon keep blood glucose balanced?

They act as opposing signals from the pancreatic islets of Langerhans: glucagon (α-cells) raises blood glucose when it's low, via glycogenolysis and gluconeogenesis; insulin (β-cells) lowers blood glucose when it's high, by promoting glucose uptake and storage. Somatostatin (δ-cells) inhibits both to prevent overcorrection.