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Endocrine Glands &
Hormone Classes

Master the major endocrine glands, their hormones, and the three chemical classes that determine how signals travel through the body.

MCAT Biological Foundations

What Is the Endocrine System?

The endocrine system is a network of ductless glands that secrete chemical messengers called hormones directly into the bloodstream. Unlike the nervous system, which sends rapid electrical signals, endocrine signaling is slower but longer-lasting, regulating processes like metabolism, growth, reproduction, and homeostasis.

The Major Endocrine Glands

Hypothalamus Pituitary Pineal Gland Thyroid Parathyroid (x4) Thymus Adrenal Glands Pancreas Ovaries Testes Kidneys (Renin, EPO)

Why Hormones Matter for the MCAT

Signal Transduction
Hormone class determines whether a signal uses intracellular receptors or membrane-bound receptors with second messengers.
Homeostasis
Negative feedback loops maintain blood glucose, calcium, and thyroid hormone within narrow ranges.
Integration
MCAT passages combine endocrine physiology with biochemistry, genetics, and organ systems.

The Three Hormone Classes

Peptide/Protein Amino acid chains Water-soluble Membrane receptors Second messengers EXAMPLES Insulin GH, ADH, Oxytocin FSH, LH, TSH Steroid Cholesterol-derived Lipid-soluble Intracellular receptors Gene transcription EXAMPLES Cortisol, Aldosterone Estrogen, Progesterone Testosterone Tyrosine-Derived Modified amino acid Varies by type Catecholamines: membrane Thyroid: intracellular EXAMPLES Epinephrine, NE Dopamine
\( T_3 \), \( T_4 \) (Thyroid)

Peptide Hormone Signaling

CELL MEMBRANE CYTOPLASM H Peptide Receptor G-protein Enzyme cAMP Second Messenger Cell Response

Steroid Hormone Signaling

CELL MEMBRANE S Steroid Diffuses Receptor H-R Complex NUCLEUS Gene Transcription mRNA → Protein

Tyrosine-Derived: Two Sub-classes

Catecholamines
Epinephrine, norepinephrine, dopamine. Act like peptide hormones: water-soluble, bind membrane receptors, use second messengers (cAMP). Fast-acting.
Thyroid Hormones
\( T_3 \) and \( T_4 \). Act like steroid hormones: lipid-soluble, cross the membrane, bind intracellular nuclear receptors, alter gene transcription. Slow-acting.

Water-Soluble vs Lipid-Soluble

Water-Soluble Lipid-Soluble
Types Peptide, Catecholamines Steroid, Thyroid (\( T_3 / T_4 \))
Transport Dissolved freely in blood Bound to carrier proteins
Receptor Location Cell surface Intracellular / Nuclear
Mechanism Second messengers (cAMP, \( \text{IP}_3 \)) Direct gene transcription
Speed of Action Fast (seconds to minutes) Slow (hours to days)
Duration Short-lived Long-lasting
Storage In vesicles, released by exocytosis Synthesized on demand

The Hypothalamic-Pituitary Axis

Hypothalamus Releasing hormones Anterior Pituitary Posterior Pituitary Thyroid (TSH) Adrenals (ACTH) Gonads (FSH/LH) ADH Oxytocin Negative Feedback Also: GH, Prolactin

How to Identify Hormone Class

1
Ask: Is it derived from cholesterol? If yes, it is a steroid (cortisol, aldosterone, sex hormones). Think "gonads and adrenal cortex."
2
Ask: Is it a chain of amino acids? If yes, it is a peptide/protein hormone (insulin, GH, FSH). Most hormones fall here.
3
Ask: Is it from the adrenal medulla or thyroid? These are tyrosine-derived. Then determine sub-class: catecholamine (fast, membrane) or thyroid (slow, nuclear).
4
Predict the mechanism. Class tells you receptor location, speed, and whether second messengers or gene transcription are involved.

Example 1: Insulin Signaling

Scenario: Blood glucose rises after a meal. The pancreatic beta cells of the islets of Langerhans release insulin. What class is insulin, and how does it work?

1
Classification: Insulin is a peptide hormone (51 amino acids, two polypeptide chains linked by disulfide bonds).
2
Receptor: Binds a receptor tyrosine kinase (RTK) on the cell surface: an exception to the typical GPCR/cAMP pathway.
3
Effect: Triggers GLUT4 transporter insertion into the membrane, increasing glucose uptake. Also promotes glycogenesis and lipogenesis.

Example 2: Cortisol and Stress

Scenario: During prolonged stress, ACTH from the anterior pituitary stimulates the adrenal cortex to release cortisol. Trace the full axis.

1
Hypothalamus releases CRH (corticotropin-releasing hormone) → travels via portal system to anterior pituitary.
2
Anterior pituitary releases ACTH → travels in bloodstream to adrenal cortex.
3
Adrenal cortex releases cortisol (a steroid) → diffuses into target cells, binds intracellular receptor, alters gene transcription.
4
Negative feedback: High cortisol inhibits both the hypothalamus and anterior pituitary, reducing CRH and ACTH release.

Example 3: Epinephrine vs \( T_3 \)

Both are tyrosine-derived, but they behave completely differently. Compare their signaling:

Epinephrine \( T_3 \) (Thyroid)
Source Adrenal medulla Thyroid gland
Solubility Water-soluble Lipid-soluble
Receptor Cell surface (GPCR) Intracellular nuclear
Mechanism cAMP cascade Gene transcription
Speed Seconds Hours to days
Function Fight-or-flight response Basal metabolic rate

Common Mistakes

✕ All tyrosine-derived are the same
Catecholamines act like peptides (surface receptors), while thyroid hormones act like steroids (nuclear receptors). Don't lump them together.
✕ Anterior = posterior pituitary
The anterior pituitary synthesizes hormones (ACTH, TSH, GH, FSH, LH, prolactin). The posterior stores hormones made in the hypothalamus (ADH, oxytocin).
✕ Steroids use second messengers
Steroids cross the membrane and bind intracellular receptors directly. They alter gene transcription: no cAMP, no \( \text{IP}_3 \) needed.

Master Summary Table

Gland Key Hormones Class Function
Hypothalamus CRH, GnRH, TRH Peptide Regulates pituitary
Ant. Pituitary ACTH, TSH, GH, FSH, LH Peptide Tropic hormones
Post. Pituitary ADH, Oxytocin Peptide Water balance, labor
Thyroid \( T_3 \), \( T_4 \), Calcitonin Tyr / Peptide Metabolism, \( \text{Ca}^{2+} \)
Adrenal Cortex Cortisol, Aldosterone Steroid Stress, Na/K balance
Adrenal Medulla Epi, NE Tyrosine (cat.) Fight-or-flight
Pancreas Insulin, Glucagon Peptide Blood glucose
Gonads Estrogen, Testosterone Steroid Reproduction

Question 1 of 5

A hormone binds to a G-protein coupled receptor on the cell surface and activates adenylyl cyclase. Which class of hormone is this most likely?

Question 2 of 5

Which of the following hormones is derived from cholesterol and directly alters gene transcription?

Question 3 of 5

The posterior pituitary releases ADH and oxytocin. What is unique about these hormones compared to anterior pituitary hormones?

Question 4 of 5

Both epinephrine and \( T_3 \) are derived from tyrosine. Why does epinephrine act within seconds while \( T_3 \) takes hours?

Question 5 of 5

A researcher finds that a hormone circulates in the blood bound to a carrier protein and has effects lasting several days. This hormone is most likely: