The Endocrine System

5 min read Updated Apr 19, 2026

The endocrine system works alongside the nervous system to control behavior, but on a slower timescale. Nervous system = fast, targeted (neurotransmitters across a synapse). Endocrine system = slow, body-wide (hormones through the blood). Together they coordinate physiology.

Anatomical illustration of a human torso with the major endocrine glands labeled in their anatomical locations: pineal gland and pituitary gland in the brain (with thalamus inset), thymus and thyroid (with parathyroids on its posterior surface) in the chest and neck, adrenal glands sitting atop each kidney, pancreas in the abdomen, and gonads (ovaries in females, testes in males) in the pelvis — The major endocrine glands and their anatomical positions. Each gland releases hormones into the bloodstream that travel body-wide to act on tissues bearing the matching receptor. *Credit: OpenStax College via Wikimedia Commons (CC BY 3.0).*

Hormones: Three Types

Peptide/protein hormones. Insulin, growth hormone, oxytocin. Water-soluble, bind cell-surface receptors, trigger second-messenger cascades inside the cell.
Steroid hormones. Testosterone, estrogen, progesterone, cortisol. Lipid-soluble, pass directly through the plasma membrane, bind intracellular receptors that act as transcription factors.
Tyrosine derivatives. Thyroid hormones (T3, T4) and catecholamines (epinephrine, norepinephrine). Made from the amino acid tyrosine.

The Hypothalamus-Pituitary Axis

The hypothalamus is the bridge between the nervous and endocrine systems. It controls the pituitary gland (“master gland”), which sits just below it.

The hypothalamus-pituitary complex. The hypothalamus releases hormones (releasing/inhibiting factors) that travel down the infundibulum to the anterior pituitary; the posterior pituitary stores and releases hormones synthesized in the hypothalamus itself. Credit: OpenStax College via Wikimedia Commons (CC BY 3.0).

Anterior Pituitary

Releases hormones in response to releasing factors from the hypothalamus. Mnemonic: FLAT PEG:

FSH - follicle-stimulating hormone (gonads)
LH - luteinizing hormone (gonads)
ACTH - adrenocorticotropic hormone (adrenal cortex → cortisol)
TSH - thyroid-stimulating hormone (thyroid)
Prolactin - milk production
Endorphins - pain modulation
GH - growth hormone (body growth, triggers adolescent growth spurt)

Posterior Pituitary

Releases hormones produced in the hypothalamus (doesn’t make its own). Two main hormones:

ADH (antidiuretic hormone/vasopressin) - water retention by the kidneys.
Oxytocin - uterine contraction during labor, milk letdown, and social bonding.

Other Endocrine Glands

Thyroid

T3 and T4 (triiodothyronine and thyroxine) regulate metabolic rate and growth.
Hypothyroidism → fatigue, weight gain, depression.
Hyperthyroidism → anxiety, weight loss, heart palpitations.
Controlled by TSH from the anterior pituitary.

Parathyroid

Four glands on the back of the thyroid.
Release parathyroid hormone (PTH) to regulate calcium levels.

Adrenal Glands

Each sits on top of a kidney. Two layers, different hormones:

Adrenal medulla (inner) - releases catecholamines: epinephrine and norepinephrine. Fast stress response.
Adrenal cortex (outer) - releases steroids including cortisol (glucocorticoid). Slower, sustained stress response. Also releases aldosterone (mineralocorticoid, salt/water balance) and sex steroids.

Mnemonic: “Cortisol from the CORTex.”

Gonads

Ovaries: estrogen and progesterone; also produce eggs.
Testes: testosterone; also produce sperm.
Stimulated by LH and FSH from the anterior pituitary.

Pancreas

Insulin (lowers blood glucose) and glucagon (raises blood glucose).
Unusually, the pancreas is NOT controlled by the pituitary. Its hormone release is triggered directly by blood glucose.

Negative Feedback Loops

Most endocrine regulation works by negative feedback - the end product of a pathway inhibits its own production. Classic example, thyroid regulation:

Hypothalamus → TRH → anterior pituitary
Anterior pituitary → TSH → thyroid
Thyroid → T3, T4 → tissues
T3 and T4 inhibit TRH and TSH → loop turns itself off

This stable feedback keeps hormone levels within a narrow range. When the loop breaks (e.g., autoimmune destruction of the thyroid), the feedback fails, and levels of TSH skyrocket trying to rescue T3/T4 output.

Diagram of the hypothalamic-pituitary-adrenal (HPA) axis as a vertical chain: hypothalamus → CRH → anterior pituitary → ACTH → adrenal cortex → cortisol. Two negative-feedback arrows from the cortisol box at the top point back to the hypothalamus and the anterior pituitary, marked with inhibitory bars — The HPA axis as the prototypical negative-feedback endocrine loop. Cortisol from the adrenal cortex feeds back to inhibit both the hypothalamus (CRH) and the anterior pituitary (ACTH), keeping its own levels within a narrow range. The same logic governs the thyroid axis (TRH → TSH → T3/T4) and the gonadal axis. *Credit: Biolinkage Project via Wikimedia Commons (CC BY-SA 4.0).*

Positive feedback loops do exist but are rare. Example: oxytocin during labor - uterine stretching triggers oxytocin release, which triggers more contractions, which stretch more, until delivery. Then the loop stops.

Name the seven hormones released by the anterior pituitary (FLAT PEG).

Click to reveal answer

FSH, LH, ACTH, TSH, Prolactin, Endorphins, Growth hormone. Posterior pituitary releases ADH and Oxytocin (made in the hypothalamus).

Which adrenal region releases cortisol, and which releases epinephrine?

Click to reveal answer

Adrenal CORTEX (outer) = cortisol (and other steroids). Adrenal MEDULLA (inner) = catecholamines (epinephrine, norepinephrine). Cortisol from the CORTex.

How does negative feedback keep thyroid hormones stable?

Click to reveal answer

Hypothalamus TRH → anterior pituitary TSH → thyroid T3/T4. Circulating T3 and T4 inhibit TRH and TSH, turning off their own production. Keeps hormones within a narrow range.