Endocrine Organs and Hormones
High-Yield Summary
- Hypothalamus controls the anterior pituitary via tropic releasing hormones (GnRH, GHRH, TRH, CRF) — except prolactin, which is inhibited by dopamine rather than stimulated.
- Anterior pituitary makes 7 hormones (mnemonic FLAT PEG): 4 tropic (FSH, LH, ACTH, TSH), 3 direct (Prolactin, Endorphins, Growth hormone).
- Posterior pituitary stores/releases oxytocin and ADH — both actually synthesized by hypothalamic neurons, not the pituitary itself.
- Thyroid: T₃/T₄ raise metabolic rate; calcitonin lowers blood calcium. Parathyroid: PTH raises blood calcium. Calcitonin and PTH are direct opposites.
- Adrenal cortex (steroid hormones): glucocorticoids (cortisol, ACTH-driven), mineralocorticoids (aldosterone, RAAS-driven), weak sex hormone precursors. Adrenal medulla: catecholamines (fight-or-flight).
- Pancreatic islets: glucagon (α-cells, raises glucose), insulin (β-cells, lowers glucose), somatostatin (δ-cells, inhibits both) — imbalance underlies Type 1/2 diabetes.
FLAT PEG — Anterior Pituitary Hormones
- F — FSH: gonads. Females: ovarian follicle growth. Males: supports Sertoli cells/sperm production. (Tropic)
- L — LH: gonads. Females: triggers ovulation, stimulates progesterone from corpus luteum. Males: stimulates Leydig cells → testosterone. (Tropic)
- A — ACTH: adrenal cortex, stimulates glucocorticoid (cortisol) release. (Tropic)
- T — TSH: thyroid, stimulates T₃/T₄ production. (Tropic)
- P — Prolactin: mammary glands, promotes milk production; regulated by dopamine INHIBITION, not a releasing hormone. (Direct)
- E — Endorphins: nervous system, natural painkillers, "runner's high." (Direct)
- G — Growth hormone: bone/muscle/liver, stimulates growth + IGF release from liver. (Direct)
Calcitonin vs. Parathyroid Hormone
| Calcitonin (Thyroid) | Parathyroid Hormone (Parathyroid Glands) |
|---|---|
| Lowers blood calcium | Raises blood calcium |
| Increases renal calcium excretion | Increases renal calcium reabsorption |
| Decreases gut calcium absorption | Activates vitamin D → increases gut calcium absorption |
| Increases calcium storage in bone | Stimulates bone resorption (releases calcium into blood) |
Insulin vs. Glucagon
| Insulin (β-cells) | Glucagon (α-cells) |
|---|---|
| Lowers blood glucose (after eating) | Raises blood glucose (fasting/between meals) |
| Stimulates glucose uptake into muscle/fat | Glycogenolysis — breaks down glycogen into glucose |
| Glycogenesis — stores glucose as glycogen | Gluconeogenesis — makes new glucose from non-carb sources |
| Promotes lipogenesis + protein synthesis | Promotes lipolysis + protein catabolism |
RAAS (Renin-Angiotensin-Aldosterone System)
- 1Low blood pressure/volume triggers renin release from the kidneys.
- 2Renin converts angiotensinogen (from liver) into angiotensin I.
- 3ACE (mainly in the lungs) converts angiotensin I into angiotensin II.
- 4Angiotensin II causes vasoconstriction and stimulates aldosterone release from the adrenal cortex.
- 5Aldosterone promotes sodium/water retention in the kidneys, restoring blood pressure and volume.
Common MCAT Trap
- Prolactin is the FLAT PEG exception — it's suppressed by dopamine (prolactin-inhibiting factor), not triggered by a releasing hormone.
- Growth hormone dysregulation depends on timing: excess before growth plates close → gigantism; excess after → acromegaly (bones don't lengthen, soft tissue/jaw/hands/feet thicken).
- Aldosterone is NOT primarily ACTH-regulated (unlike cortisol) — it's controlled by RAAS.
- Type 1 diabetes = no insulin (autoimmune β-cell destruction, childhood onset); Type 2 = insulin resistance (later onset, insulin normal/elevated at first).
Quick Recall
Which anterior pituitary hormones are tropic vs. direct?
What hormone does the posterior pituitary actually synthesize?
Which adrenal cortex hormone is regulated by RAAS instead of ACTH?
What does somatostatin do in the pancreatic islets?