Endocrine System: Mechanisms of Hormone Action

The endocrine system is a long-distance chemical messaging network. Unlike the nervous system's fast, point-to-point electrical signals, the endocrine system releases hormones into the bloodstream to broadcast them body-wide. The MCAT challenge is understanding how a hormone finds the right cell, what it does there, and how the signal is turned off.

Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.

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Hormone Classification by Chemical Nature

Must know

A hormone's chemical nature determines where its receptor lives and what it does. This is the single most important organizing idea for the topic.

Peptide and Protein Hormones

Must know

Chains of amino acids. Because they are polar and hydrophilic, they cannot cross the lipid bilayer — they bind cell-surface receptors and rely on second messengers to relay the signal inward. They are pre-made and stored in vesicles, released on demand.

Passage-level

Insulin/glucagon (pancreas); ADH/oxytocin (posterior pituitary, made in hypothalamus); all hypothalamic releasing/inhibiting hormones and anterior pituitary tropic hormones (GH, TSH, ACTH, FSH, LH, prolactin).

Steroid Hormones

Must know

Derived from cholesterol; nonpolar and lipophilic, synthesized on demand (not stored). They diffuse through the membrane and bind intracellular receptors; the hormone-receptor complex acts as a transcription factor, altering gene expression. Effects are slow (hours) but long-lasting.

Passage-level

Cortisol, aldosterone (adrenal cortex); testosterone, estradiol, progesterone (gonads/placenta); calcitriol (active vitamin D, steroid-like).

Amino Acid Derivatives (Tyrosine-Derived)

Must know

This class splits into two opposite behaviors:

Hormone	Solubility	Receptor Location
Epinephrine, norepinephrine (catecholamines)	Hydrophilic	Plasma membrane
Thyroid hormones ($\ce{T3}$, $\ce{T4}$)	Lipophilic	Nuclear

Thyroid hormones behave like steroids (nuclear receptor, alter transcription) even though they come from an amino acid. This exception is heavily tested.

Quick check: A researcher blocks all plasma membrane receptors on a target cell. Which hormones still work?

Answer: Lipophilic hormones — steroids (cortisol, testosterone, estrogen, progesterone) and thyroid hormones ($\ce{T3/T4}$) — which enter the cell and bind intracellular/nuclear receptors. Peptides and catecholamines are blocked.

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Transport of Hormones: Blood Supply

Free vs. Bound Hormone

Must know

Hormones travel free (dissolved) or bound to carrier proteins.

Only free hormone is biologically active. Bound hormone is an inactive reservoir that extends half-life by protecting against degradation and renal clearance.

Know the logic

Lipophilic hormones (steroids, thyroid) need carriers to stay soluble in plasma and have long half-lives; peptides/catecholamines circulate free and are short-lived (degraded in minutes).

Optional

Albumin (high-capacity, low-affinity, carries many steroids/thyroid hormone), SHBG (testosterone, estradiol), TBG (T3/T4), CBG/transcortin (cortisol).

Clinical hook: Estrogen states (pregnancy, oral contraceptives) raise carrier protein levels. This raises total hormone (e.g., total $\ce{T4}$ or total cortisol) while free hormone stays normal — so the patient is still euthyroid/normal. Measuring only total levels would mislead.

Portal Systems

Must know

The hypothalamic-pituitary portal system carries hypothalamic releasing hormones a short distance directly to the anterior pituitary, reaching high local concentrations without systemic dilution.

Quick check: A patient has normal free cortisol but high total cortisol. Why?

Answer: Elevated corticosteroid-binding globulin (CBG), often from high-estrogen states. More cortisol is bound, raising total, while the active free fraction is normal.

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Specificity of Hormones: Target Tissue

Must know

A hormone reaches every cell, yet only some respond. Specificity comes from receptor expression — a cell responds only if it expresses the cognate receptor.

Receptor location follows the hormone's lipophilicity:

• Hydrophilic hormones → membrane receptors (GPCRs, RTKs)
• Lipophilic hormones → nuclear receptors

Receptor Regulation

Know the logic

Cells tune sensitivity by changing receptor number.

• Downregulation: prolonged high hormone → fewer receptors → less sensitive. Classic example: chronic hyperinsulinemia downregulates insulin receptors, contributing to insulin resistance.
• Upregulation: low hormone → more receptors → more sensitive (e.g., estrogen upregulates uterine progesterone receptors to "prime" the uterus).

Permissiveness, Synergism, Antagonism

Know the logic

• Permissiveness: hormone A must be present for B's full effect (thyroid hormone permits epinephrine's cardiac effect).
• Synergism: two hormones together exceed the sum of each alone.
• Antagonism: one opposes another (insulin vs. glucagon).

Quick check: Why do users of exogenous testosterone often get testicular atrophy?

Answer: Exogenous testosterone exerts negative feedback on the hypothalamus/pituitary, suppressing GnRH → LH/FSH. Without LH, Leydig cells stop producing testosterone and the testes shrink from disuse.

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Integration with the Nervous System: Feedback Control

Must know

The hypothalamus is the neuroendocrine interface: it converts neural input into hormonal output.

The Hypothalamic–Pituitary–Target Gland Axis

Must know

$\text{Hypothalamus} \xrightarrow{\text{releasing hormone}} \text{Anterior Pituitary} \xrightarrow{\text{tropic hormone}} \text{Target Gland} \xrightarrow{\text{hormone}} \text{Target Tissue}$

Axis	Hypothalamus	Pituitary	End Hormone
Thyroid	TRH	TSH	$\ce{T3/T4}$
Adrenal	CRH	ACTH	Cortisol
Gonadal	GnRH	LH, FSH	Testosterone/Estrogen/Progesterone
Growth	GHRH / Somatostatin	GH	IGF-1 (liver)

Note the distinction between tropic hormones (act on other endocrine glands, e.g., TSH, ACTH) and direct/non-tropic hormones (act on non-endocrine tissue).

Negative Feedback (the dominant mode)

Must know

The end product inhibits earlier steps, holding levels in range. Example: $\ce{T3/T4}$ suppress both TRH and TSH. If thyroid hormone falls, inhibition lifts and TSH rises — which is why elevated TSH is the marker of primary hypothyroidism.

Positive Feedback (the exceptions)

Must know

Positive feedback amplifies the stimulus; it is rare. The two tested examples:

1. LH surge: high estrogen from the dominant follicle switches from negative to positive feedback at mid-cycle, triggering ovulation.
2. Oxytocin in labor: contractions → oxytocin → stronger contractions, a self-amplifying loop.

Neural Override: the Adrenal Medulla

Know the logic

In fight-or-flight, the hypothalamus drives direct sympathetic neural stimulation of the adrenal medulla (a modified sympathetic ganglion), which releases epinephrine/norepinephrine in seconds — bypassing the slower pituitary relay.

Worked example: A pituitary adenoma autonomously secretes excess ACTH. Predict CRH, ACTH, cortisol.

• Cortisol ↑↑ — chronic ACTH drives the adrenal cortex.
• ACTH ↑ — the adenoma ignores feedback.
• CRH ↓ — high cortisol feeds back on the hypothalamus.

This is Cushing's disease (high ACTH + high cortisol). Contrast: a primary adrenal tumor gives high cortisol but low ACTH (feedback suppression) — a key distinction.

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Regulation by Second Messengers

Must know

When a hydrophilic hormone binds its surface receptor, the message must reach the cell interior without the hormone crossing the membrane. Second messengers carry it inward.

G Protein–Coupled Receptors (GPCRs)

Must know

GPCRs are the most-tested receptor class. A bound hormone activates a heterotrimeric G protein ($\alpha\beta\gamma$); the G$\alpha$ subunit swaps GDP for GTP, dissociates, and acts on downstream effectors. Three functional flavors:

G Protein	Effect	Example
$G_s$	↑ cAMP (activates adenylyl cyclase)	Glucagon, epi (β), ADH (V2), PTH
$G_i$	↓ cAMP (inhibits adenylyl cyclase)	Epi (α₂), somatostatin
$G_q$	Activates PLC → IP₃/DAG	Oxytocin, ADH (V1)

The cAMP Pathway ($G_s$ / $G_i$)

Must know

$\text{Hormone} \to \text{GPCR} \to G_s \to \textbf{Adenylyl cyclase} \to \ce{ATP -> cAMP} \to \textbf{PKA} \to \text{phosphorylation of targets}$

PKA phosphorylates serine/threonine residues. Signal is terminated by phosphodiesterase (PDE), which degrades cAMP to 5'-AMP (caffeine inhibits PDE, prolonging signaling).

Signal Amplification

Know the logic

Each cascade step multiplies the signal, so one hormone molecule produces thousands of downstream events. A single epinephrine-receptor event can yield ~10,000 phosphorylations within a second (one adenylyl cyclase → many cAMP → many PKA → many substrates); multi-step cascades like liver glycogen breakdown reach $>10^6$-fold. Amplification = the product of the rates at each step. You do not need to memorize specific numbers — just the multiplicative principle.

The IP₃/DAG Pathway ($G_q$)

Must know

$G_q$ activates phospholipase C (PLC), which cleaves $\ce{PIP2}$ into:

1. IP₃ — soluble; opens ER calcium channels, releasing $\ce{Ca^{2+}}$.
2. DAG — stays in the membrane; activates protein kinase C (PKC).

$\ce{Ca^{2+}}$ is itself a second messenger, often acting via calmodulin. Oxytocin and ADH (V1) use this pathway.

The cGMP Pathway

Know the logic

Nitric oxide (NO), made from arginine by NO synthase, is small/lipophilic, crosses membranes, and activates soluble guanylyl cyclase → cGMP → PKG → smooth-muscle vasodilation (how nitroglycerin works; sildenafil inhibits the PDE that degrades cGMP). Optional ANP uses a membrane-bound guanylyl-cyclase receptor to raise cGMP and promote natriuresis/vasodilation.

Receptor Tyrosine Kinases (RTKs): The Insulin Receptor

Must know

The insulin receptor is the canonical RTK and has intrinsic kinase activity.

1. Insulin binds the extracellular domain (a pre-dimerized $\alpha_2\beta_2$ receptor).
2. Binding triggers autophosphorylation of tyrosine residues on the $\beta$ subunits.
3. The receptor recruits/phosphorylates substrates (e.g., IRS-1).
4. Downstream signaling (PI3K → Akt) drives GLUT4 translocation, enabling glucose uptake in muscle/adipose.

Key contrast: RTKs phosphorylate tyrosine; PKA/PKC phosphorylate serine/threonine.

Intracellular Receptors: Steroid and Thyroid Hormones

Must know

Lipophilic hormones diffuse in and bind cytosolic or nuclear receptors. The hormone-receptor complex dimerizes, binds hormone response elements (HREs) on DNA, and modulates transcription. This is a direct genomic effect — no second messenger, slow onset, long-lasting (effects outlast the hormone).

Quick check: A drug blocks phospholipase C. Which pathway is most disrupted: (A) epinephrine via β-receptor, (B) oxytocin on uterine muscle, or (C) cortisol on liver?

Answer: (B). Oxytocin uses $G_q$ → PLC → IP₃/DAG. (A) uses $G_s$/cAMP, not PLC. (C) cortisol is a steroid using a nuclear receptor — no second messenger.

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Common Confusions & Tricks

1. Thyroid hormones are NOT peptides — they are lipophilic. $\ce{T3/T4}$ are iodinated tyrosine derivatives that use nuclear receptors like steroids. For mechanism questions, think transcription, not second messenger.

2. Epinephrine's effect depends on the receptor. It can bind α₁ ($G_q$, vasoconstriction), α₂ ($G_i$, ↓cAMP), or β ($G_s$, ↑cAMP). Same hormone, different/opposite effects by tissue.

3. Elevated TSH = hypo, not hyper. High TSH → primary hypothyroidism; low TSH → primary hyperthyroidism.

4. ADH uses two receptors. V2 (renal collecting duct) → $G_s$ → cAMP → aquaporin-2 (water retention). V1 (vascular smooth muscle) → $G_q$ → vasoconstriction.

5. "Only free hormone is active." Elevated TBG (estrogen, pregnancy, OCPs) raises total $\ce{T4}$ but the patient is euthyroid because free $\ce{T4}$ is unchanged.

6. Positive feedback is rare and transient. The LH surge is positive feedback only at mid-cycle high estrogen; at lower estrogen, estrogen is still negative feedback.

7. RTKs phosphorylate tyrosine; PKA/PKC phosphorylate serine/threonine.

8. $G_q$ map: $G_q$ → PLC → PIP₂ → IP₃ + DAG → $\ce{Ca^{2+}}$ + PKC. IP₃ goes to the ER (release calcium); DAG stays in the membrane (activate PKC).

9. Downregulation explains insulin resistance. Chronic hyperinsulinemia → receptor downregulation → less responsive cells → more insulin secretion → vicious cycle.

10. The adrenal medulla is neural in its input. Directly innervated by sympathetic fibers; releases epinephrine in seconds without waiting for the pituitary axis.

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Key Mechanisms & Relationships

There are no equations to memorize; the relationships below are the signaling schemes that capture how each receptor class transduces its signal.

Mechanism / Relationship	When to Use
$[\text{Hormone}]_\text{free} = [\text{Total}] - [\text{Bound}]$	Only free hormone acts on target tissue
$\ce{ATP ->[\text{adenylyl cyclase}] cAMP + PPi}$	Gs activation; cAMP → PKA pathway
$\ce{cAMP ->[\text{PDE}] 5'-AMP}$	PDE terminates cAMP signaling (caffeine inhibits)
$\ce{PIP2 ->[\text{PLC}] IP3 + DAG}$	Gq activation; IP₃ releases $\ce{Ca^{2+}}$, DAG activates PKC
$\ce{GTP ->[\text{guanylyl cyclase}] cGMP + PPi}$	NO (soluble GC) or ANP (receptor GC) → PKG → vasodilation
$\text{RTK} + \text{ligand} \to \text{autophosphorylation (Tyr-P)}$	Insulin receptor; intrinsic kinase phosphorylates tyrosine
$[\text{Hormone-Receptor}] \cdot \text{HRE} \to \Delta \text{transcription}$	Steroid/thyroid nuclear receptor; direct genomic effect