Endocrine System

Type 1's absolute insulin deficiency and type 2's insulin resistance both make sense against insulin's normal role as the body's dominant anabolic, glucose-clearing hormone, covered in Insulin & Glucagon → Insulin.

A1c, fasting glucose, and the OGTT are measuring the same underlying pathology at different time resolutions — A1c integrates ~3 months of glycemic exposure, while the other two are single-moment snapshots — which is why any one of them alone is diagnostic, but they can disagree in a given patient.

Nearly every row here traces back to one fork — autoimmune β-cell destruction (absolute deficiency, ketosis-prone) versus resistance plus progressive dysfunction (relative deficiency, ketosis-resistant) — so a feature that doesn't obviously follow from that split, like HLA association, is worth flagging as the exception rather than memorizing the table as a flat list.

• In tissues that take up glucose independent of insulin — the lens, retina, peripheral nerves, and kidney — the enzyme aldose reductase converts excess intracellular glucose into sorbitol. These same tissues have relatively little sorbitol dehydrogenase, the enzyme that would normally break sorbitol down further, so it accumulates and osmotically pulls water into the cell. This osmotic swelling is a distinct mechanism from vascular glycation damage and is the specific explanation given for diabetic cataracts and contributes to peripheral neuropathy.

DKA's ketogenic cascade begins because insulin normally suppresses lipolysis — that antilipolytic action is detailed in Insulin & Glucagon → Insulin.

DKA and HHS sit on the same spectrum of insulin deficiency — DKA patients (type 1) have essentially none, so lipolysis runs unchecked into ketogenesis, while HHS patients (type 2) retain just enough residual insulin to block lipolysis but not enough to control glucose, trading ketoacidosis for more extreme hyperglycemia and dehydration instead.

• Fluids first — isotonic saline to correct volume depletion (essential in both, especially HHS)
• Insulin — halts lipolysis/ketogenesis and drives glucose into cells; hold until potassium is known to be safe
• Potassium repletion — total body potassium is depleted even though initial serum levels often look normal or high, because acidosis shifts K⁺ out of cells; insulin therapy will drive it back in and can precipitate dangerous hypokalemia if not replaced

Sulfonylureas and meglitinides work by directly closing the ATP-sensitive K⁺ channel that normally triggers insulin release only in response to rising glucose, a mechanism walked through in Insulin & Glucagon → How a β Cell Actually Decides to Release Insulin.

Agents that force insulin release (sulfonylureas, meglitinides) only work with a functioning β-cell mass, while agents that improve sensitivity or block absorption (metformin, TZDs, acarbose, SGLT2 inhibitors) don't depend on β cells at all — that split is exactly why the first group is useless in type 1 diabetes and carries a hypoglycemia risk the second group largely avoids.

• Metformin is first-line for type 2 diabetes largely because it does not cause hypoglycemia on its own and tends to promote modest weight loss rather than gain

Insulin and glucagon are built as functional opposites at nearly every level — anabolic vs. catabolic, tyrosine kinase vs. Gs-cAMP signaling, triggered by rising vs. falling glucose — so most exam questions about one hormone's action can be answered by inverting what you know about the other.

• Sulfonylureas and meglitinides work by directly closing that same ATP-sensitive K⁺ channel, forcing depolarization and insulin release independent of the actual glucose level — which is exactly why they can cause hypoglycemia in a way that metformin or SGLT2 inhibitors cannot

GLUT4 is the only tissue-restricted, insulin-dependent transporter — brain, RBCs, liver, and gut take up glucose regardless of insulin status — which is why the brain never experiences the profound cellular glucose starvation seen in muscle and fat during diabetes, and why hypoglycemia's danger is specifically neuroglycopenic rather than affecting insulin-dependent tissues first.

Cortisol excess is one of the listed causes of hyperglycemia because of its gluconeogenic and anti-insulin actions, spelled out in Adrenal Cortex Disease → What Cortisol Actually Does, Beyond "Stress Hormone".

• Documented low plasma glucose
• Symptoms consistent with hypoglycemia at that time
• Resolution of symptoms once glucose is corrected

GLP-1 agonists appear here for weight loss using the same incretin-mimetic mechanism used to treat diabetes, detailed in Diabetes Pharmacology → Oral & Injectable Agents by Class.

These three drugs hit three different points in energy balance — gut fat absorption (orlistat), central appetite/adrenergic drive (phentermine), and incretin-mediated satiety plus gastric emptying (GLP-1 agonists) — so their side-effect profiles predictably track their mechanism: steatorrhea, cardiovascular stimulation, and nausea, respectively.

• Persistently high aldosterone would be expected to cause progressive fluid overload, but the kidney counters it with an "aldosterone escape" — rising blood volume triggers natriuretic peptides (ANP/BNP) that push back against aldosterone's sodium-retaining effect once volume expansion reaches a certain point, capping further fluid gain. This is why Conn syndrome classically presents with hypertension but no edema. In heart failure, though, that same escape mechanism is itself impaired by poor renal perfusion, so aldosterone-driven fluid retention continues unchecked and worsens the existing edema.

• Cushing disease = pituitary ACTH excess specifically → high ACTH, and can cause hyperpigmentation like Addison disease
• Cushing syndrome = umbrella term for any cause of hypercortisolism; adrenal-source and iatrogenic causes have low ACTH due to feedback suppression

• At very high concentrations — as seen with ectopic ACTH syndrome (e.g., small cell lung cancer) — cortisol overwhelms the enzyme that normally protects the mineralocorticoid receptor from being activated by cortisol, and starts binding that receptor directly. The result is a picture with prominent hypokalemia and hypertension layered on top of the usual Cushing features, mimicking primary hyperaldosteronism even though aldosterone itself is not the driver.

The workup deliberately separates "is cortisol high" from "where is it coming from" — ACTH splits ACTH-independent (adrenal/exogenous) from ACTH-dependent causes, and only then does high-dose dexamethasone distinguish a still feedback-sensitive pituitary tumor from a feedback-resistant ectopic source — so jumping straight to imaging without this sequence risks imaging the wrong organ.

Cortisol's effects all flow from one survival logic — mobilize fuel, let catecholamines work more effectively, and dial back "expensive" processes like inflammation and collagen turnover that aren't needed for immediate survival — which is why chronic excess produces a coherent syndrome (hyperglycemia, hypertension, thin skin, osteoporosis) rather than a random grab-bag of findings.

CAH's ACTH-driven hyperplasia only makes sense once you know what cortisol normally does and how it feeds back on the pituitary, covered in Adrenal Cortex Disease → What Cortisol Actually Does, Beyond "Stress Hormone".

• Virilized + hypotensive/salt-wasting → think 21-hydroxylase
• Virilized + hypertensive → think 11β-hydroxylase
• Sexually infantile + hypertensive → think 17α-hydroxylase

Pheochromocytoma's association with MEN 2A/2B reflects a shared germline RET mutation with medullary thyroid carcinoma, laid out in Multiple Endocrine Neoplasia Syndromes.

• ~10% bilateral, ~10% malignant, ~10% extra-adrenal (paraganglioma), ~10% familial/syndromic, ~10% occur in children

• Carcinoid tumors are confirmed with a 24-hour urine collection for 5-HIAA, the main breakdown product of serotonin
• Zollinger-Ellison syndrome is confirmed by the secretin stimulation test — giving secretin normally suppresses gastrin release, but a gastrinoma keeps secreting gastrin anyway (paradoxical rise)
• All of these tumors can share a common biomarker, chromogranin A (and often synaptophysin), since both are stored and released alongside hormone from neuroendocrine secretory granules regardless of which specific hormone is overproduced
• Octreotide scintigraphy (a radiolabeled somatostatin analog scan) can localize and stage many of these tumors, since most neuroendocrine tumor cells are studded with somatostatin receptors

• Roughly a third metastasize, a third occur alongside a second, unrelated malignancy, and a third are multiple at diagnosis — a useful way to remember that carcinoid tumors are more biologically unpredictable than their sometimes-indolent reputation suggests

Glucocorticoid and fludrocortisone replacement here are direct treatments for the cortisol and aldosterone deficiency of Addison disease, covered in Adrenal Cortex Disease.

Desmopressin, conivaptan/tolvaptan, and demeclocycline all act on the same V2-receptor/collecting-duct axis but in opposite directions — DDAVP mimics ADH for central DI, while the other two blunt or block ADH action for SIADH — so matching drug to disorder is really just matching the direction of water handling that's needed.

Iodine-induced hyperthyroidism (Jod-Basedow) is the mirror image of the Wolff-Chaikoff effect, both grounded in the hormone synthesis pathway covered in Thyroid Hormone Synthesis.

Hashimoto and postpartum thyroiditis both share a destructive-release mechanism that can transiently look hyperthyroid before settling into hypothyroidism — the discriminator isn't the lab pattern at any single moment but the time course and whether the gland is tender, the same logic that applies to subacute thyroiditis below.

• When the thyroid itself is failing, the hypothalamus ramps up TRH to try to push more TSH out — but TRH isn't thyroid-selective, and it also stimulates prolactin release from the same anterior pituitary. The result is that uncontrolled primary hypothyroidism can present with galactorrhea and an elevated prolactin, mimicking a prolactinoma. Checking TSH is a routine part of any prolactin-elevation workup for exactly this reason.

The fastest way to sort this table is autonomous overproduction (Graves, toxic nodules — high radioiodine uptake) versus destructive hormone leak or exogenous source (subacute thyroiditis, factitious use — low uptake); uptake scanning, not the clinical picture alone, is what actually separates these two mechanistically distinct groups.

• Smooth/diffuse enlargement: Graves disease, Hashimoto thyroiditis, iodine deficiency
• Nodular enlargement: toxic multinodular goiter, toxic adenoma, thyroid neoplasm, simple thyroid cyst

• T4 is the dominant secreted product but is the less active hormone; peripheral deiodinases convert it to the far more potent T3 (roughly 35× more potent than T4)
• T4 has a long half-life (~5–7 days), which is why once-daily dosing works well for hormone replacement; T3's half-life is much shorter (~1 day)
• Most circulating T4/T3 is protein-bound to thyroxine-binding globulin (TBG) — only the free fraction is biologically active, and the pituitary's negative feedback loop only "sees" the free fraction, so a healthy axis keeps free hormone normal even when TBG itself changes
  • ↑ TBG in pregnancy and with OCP use (estrogen increases TBG production) → total T4/T3 measured on a lab panel rises, but free T4/T3 and TSH stay normal — a classic trap for mistaking a pregnant patient for hyperthyroid based on total hormone levels alone
  • ↓ TBG in nephrotic syndrome (lost in the urine along with other proteins), cirrhosis, and glucocorticoid use → total T4/T3 falls, again with a normal free fraction and normal TSH
• Wolff-Chaikoff effect — paradoxically, a large excess iodine load transiently shuts down thyroid hormone synthesis rather than boosting it, an autoregulatory protective mechanism against iodine overload

Nearly every effect listed here — heat production, tachycardia, bone growth, surfactant synthesis — is downstream of the same core action: increasing basal metabolic machinery and receptor sensitivity to other signals like catecholamines, rather than thyroid hormone doing something fundamentally different in each organ.

Medullary carcinoma's germline RET mutation is the same driver behind the pheochromocytomas of MEN 2A/2B, covered in Adrenal Medulla Tumors.

• Papillary carcinoma is linked to BRAF mutations and RET/PTC gene rearrangements
• Follicular carcinoma is linked to RAS mutations and the PAX8-PPAR-γ fusion translocation
• Medullary carcinoma is linked to germline or somatic RET mutations — the same gene disrupted in MEN 2A/2B

• A palpable thyroid nodule is worked up with fine-needle aspiration as the first step; a "cold" nodule on radioiodine scan (not taking up iodine) raises more concern for malignancy than a "hot," autonomously functioning one, though FNA cytology — not the scan — is what actually establishes the diagnosis

Methimazole and PTU both work by blocking thyroid peroxidase, and PTU additionally blocks peripheral T4→T3 conversion — both steps of the pathway covered in Thyroid Hormone Synthesis.

The choice between methimazole and PTU, or between a thionamide and radioactive iodine, usually comes down to speed versus permanence — PTU's extra T4→T3 block makes it faster-acting for a crisis or pregnancy, while radioiodine trades a slower onset for a definitive (if often hypothyroid-inducing) fix.

• PTH raises serum calcium and lowers serum phosphate
• Vitamin D raises both serum calcium and serum phosphate

• Only the free (ionized) fraction of serum calcium is biologically active; roughly 40% is bound to albumin and doesn't count toward the "active" pool
• Albumin also binds H⁺, and the two compete for the same binding sites — so alkalosis (fewer H⁺ around) frees up albumin to bind more calcium, dropping ionized calcium and triggering symptoms (perioral numbness, carpopedal spasm) even though the total calcium on a lab panel looks normal — classically seen with hyperventilation
• Acidosis works in reverse — more H⁺ competes calcium off albumin, raising ionized calcium
• This is exactly why a "corrected calcium" calculation (adjusting for albumin) matters clinically, and why a normal total calcium doesn't rule out a true ionized-calcium abnormality

Every disorder here is a variation on the PTH-driven bone/kidney/gut axis explained in Calcium Homeostasis.

• Bone pain, kidney stones, abdominal pain/constipation, and psychiatric/cognitive disturbance — the four classic clusters of a hypercalcemic state, whatever the underlying cause

• Osteitis fibrosa cystica — classically from primary hyperparathyroidism; relentless osteoclast overactivity hollows out cystic spaces in bone that fill in with fibrous tissue and old hemorrhage pigment, producing brown-appearing lesions and bone pain
• Renal osteodystrophy — the umbrella term for the bone disease that develops in chronic kidney disease, driven by the secondary (and eventually tertiary) hyperparathyroidism that CKD provokes

Cinacalcet lowers PTH by making the parathyroid gland think calcium is higher than it is, while sevelamer and the potassium binders act entirely outside the parathyroid-calcium axis in the gut — which is why a dialysis patient with secondary hyperparathyroidism often ends up on several of these at once, each solving a separate piece of the CKD-mineral picture.

Each disorder here is a break somewhere along a hypothalamic-pituitary-target axis, mapped out in Hormone Overview → Hypothalamic–Pituitary Axis at a Glance.

Most of these syndromes look identical to their non-pituitary counterparts (Cushing disease vs. syndrome, SIADH regardless of source) because the downstream hormone excess is the same — what's specific to a pituitary origin is the potential for mass effect, like bitemporal hemianopia, which can occur with a large enough adenoma of any hormone type, including a clinically silent one.

• CNS disease/head trauma, Ectopic hormone secretion by a tumor (classically small cell lung cancer), Exogenous ADH-like drugs (desmopressin, oxytocin), Lung disease (pneumonia, other infections), Drugs (SSRIs, carbamazepine, cyclophosphamide) — a broad list, but any one of these driving free-water retention leads to the same euvolemic hyponatremia

Central DI responds to desmopressin and nephrogenic DI doesn't because they involve two different points of the same ADH-receptor pathway, detailed in Hormone Overview → ADH has two separate jobs at two separate receptors.

All three polyuric conditions can look alike on a random urine sample, but the water deprivation test isolates the actual defect — failure to concentrate with dehydration alone means an ADH problem (central or nephrogenic), and only the response to exogenous desmopressin tells you which one, while primary polydipsia is the odd one out because its kidney and pituitary both work fine.

• Primary polydipsia is a behavioral cause of excess free-water intake, not a true ADH problem; it classically presents with low-normal plasma sodium/osmolality (from dilution), the opposite direction from true DI, which tends toward hypernatremia
• Nephrogenic DI can paradoxically improve with a thiazide diuretic, which induces mild volume depletion that increases proximal water/sodium reabsorption and reduces delivery of free water to the collecting duct
• DI can also be transiently induced during pregnancy — placental enzymes increase the metabolic breakdown of circulating ADH

• Almost every anterior pituitary axis is under negative feedback from the end-organ hormone back onto both the hypothalamus and the pituitary — know this loop cold, since most pituitary/target-gland pathology questions test where the loop is broken
• GnRH is the exception that proves useful clinically: continuous (non-pulsatile) GnRH exposure paradoxically suppresses LH/FSH — basis for GnRH agonist therapy in prostate cancer and precocious puberty
• Inhibin, secreted by Sertoli cells (male) and granulosa cells (female), selectively suppresses FSH without a matching effect on LH — a separate feedback arm distinct from the sex steroids

• V2 receptors (kidney collecting duct, Gs–cAMP) — ADH's primary physiologic role, inserting aquaporin-2 channels to reabsorb free water and concentrate urine
• V1 receptors (vascular smooth muscle, Gq–IP₃) — cause vasoconstriction; only becomes clinically relevant at the very high doses used therapeutically
• This V1 vasoconstrictor effect is exploited pharmacologically: synthetic vasopressin is used as a second-line pressor in refractory septic shock, working through a completely separate receptor from the one targeted by desmopressin (which is V2-selective)

LH, FSH, TSH, and hCG sharing an identical α subunit isn't just trivia — it's why extremely high hCG, as in a molar pregnancy, can cross-react enough at the TSH receptor to cause real hyperthyroidism, and why immunoassays have to target the unique β subunit specifically to avoid false cross-reactivity between these hormones.

• Finasteride — 5α-reductase inhibitor, blocks testosterone→DHT conversion; used for benign prostatic hyperplasia and male-pattern baldness
• Flutamide — competitive androgen receptor antagonist; used for prostate carcinoma and BPH
• Leuprolide — the classic GnRH agonist used clinically; continuous dosing exploits the paradoxical suppression effect for prostate cancer and central precocious puberty

• Like TBG for thyroid hormone, SHBG binds circulating testosterone and estrogen — only the unbound (free) fraction is biologically active
• In males, conditions that raise SHBG (e.g., cirrhosis, hyperthyroidism, aging) bind up more testosterone, lowering the free/active fraction relative to estrogen — this shifted ratio helps explain gynecomastia in these settings
• In females, conditions that lower SHBG (e.g., obesity, insulin resistance/PCOS) leave more testosterone unbound and active, driving hirsutism and other androgenic features
• Estrogen itself (pregnancy, OCPs) raises SHBG production by the liver, the same way it raises TBG

• Stimulated by GHRH, exercise, sleep, puberty, hypoglycemia, and endogenous opioids
• Inhibited by somatostatin, hyperglycemia, and obesity
• Somatostatin itself is not pituitary-exclusive — it's also released from the hypothalamus, the GI tract, and pancreatic δ cells, where it broadly dampens insulin, glucagon, and gastrin secretion in addition to GH
• A synthetic GHRH analog (tesamorelin) is used clinically to reduce visceral fat in HIV-associated lipodystrophy, exploiting GH's normal lipolytic effect

• Frequent breastfeeding keeps prolactin chronically elevated, and prolactin suppresses GnRH — this delays the return of ovulation postpartum and is the physiologic basis of lactational amenorrhea as a (imperfect) natural contraception method

• Ghrelin — the "I'm empty, go eat" stomach signal; secreted in a fasting state and suppressed after a meal
• Leptin — the "I have enough fat stored, stop eating" adipose tissue signal; in obesity, leptin levels are actually high but the hypothalamus becomes resistant to the signal, so appetite isn't suppressed the way it should be
• Endocannabinoids — act on cannabinoid receptors in the hypothalamus and a reward-related brain region (nucleus accumbens) to drive both hunger and the pleasurable, reward side of eating — the same pathway responsible for cannabis-induced appetite increases

• The insulin receptor is a preformed α₂β₂ tetramer — insulin binds the extracellular α subunits, triggering autophosphorylation of the intracellular tyrosine kinase domains on the β subunits

The exocrine and endocrine pancreas develop from different germ layers within the same organ — endoderm for the acinar/ductal system, mesoderm for the islets — which is the embryologic reason the two compartments have such different functions (digestive enzymes vs. hormones) despite sitting side by side.

Most of the anomalies below trace back to an embryologic origin or migration path detailed in Embryologic Development — thyroglossal duct cysts and ectopic thyroid from the thyroid's descent, absent parathyroids from pharyngeal pouch failure, and annular pancreas from abnormal ventral bud rotation.

• Neck mass location is a fast discriminator: thyroglossal duct cysts are midline; branchial cleft cysts are lateral, along the anterior sternocleidomastoid border

• Anterior = glandular tissue = makes its own hormones (six of them). Posterior = neural tissue = ships out hormones made elsewhere (two of them). If a hormone's name starts with an axis you already know terminates in a classic feedback loop (TSH, ACTH, FSH/LH, GH, prolactin), it's anterior; if it's ADH or oxytocin, it's posterior.

• GFR from outside in pairs with salt, sugar, sex — glomerulosa makes the salt-retaining hormone, fasciculata makes the sugar-raising hormone, reticularis makes the sex-steroid precursor. Going deeper, the hormone gets "more vital to acute survival" in the middle (cortisol) before shifting to reproductive physiology at the innermost layer.

The islet isn't just three cell types randomly mixed together — δ cells interspersed throughout the islet let somatostatin locally brake both insulin and glucagon release before they even reach the bloodstream, a paracrine control layer sitting on top of the systemic glucose feedback each cell type responds to independently.

• MEN 1 → three "P" organs: Pituitary, Parathyroid, Pancreas
• MEN 2A → parathyroid plus the two tumors shared with 2B (medullary thyroid carcinoma, pheochromocytoma)
• MEN 2B → same two tumors as 2A, swap parathyroid involvement for mucosal neuromas and a marfanoid habitus