Feedback Loops & Fails
This hormone drives uterine contractions during childbirth and is a classic example of endocrine positive feedback.
What is oxytocin?
A 52-year-old sedentary male with a BMI of 34 presents for a routine checkup. He notes mild fatigue and slow-healing cuts. Laboratory tests reveal a fasting plasma glucose of 142 mg/dL and a hemoglobin A1c of 7.2%, with no serum autoantibodies detected.
What is diabetes type 2?
This form of signaling occurs when a cell releases a chemical signal that binds to receptors on nearby neighboring cells.
What is paracrine signaling?
This peptide hormone, structurally related to glucagon, is secreted by L-cells in the distal gut and is the target of blockbuster weight-loss drugs like semaglutide.
What is GLP-1 (Glucagon-like peptide-1)?
This biguanide medication is the preferred first-line agent for Type 2 diabetes because it reduces hepatic glucose production, increases peripheral insulin sensitivity, and carries a low risk of hypoglycemia.
What is Metformin?
A patient has exophthalmos, weight loss, heat intolerance, and increased T3 and T4.
What is Graves' Disease?
A 19-year-old male presents with gynecomastia, small testes, and a tall stature with long limbs. Labs show elevated LH and FSH but low testosterone. Karyotype confirms 47,XXY. What does he have?
What is Klinefelter Syndrome?
The receptor for this metabolic hormone utilizes a pre-dimerized receptor tyrosine kinase that autophosphorylates upon ligand binding.
What is the insulin receptor?
Secreted by the stomach, this "hunger hormone" binds to the growth hormone secretagogue receptor to stimulate appetite.
What is Ghrelin?
This definitive treatment for Graves' disease leverages the thyroid gland’s unique affinity for a specific halogen to permanently ablate overactive follicular cells, but requires patients to strictly avoid close contact with children and pregnant women for several days post-dose.
What is Radioactive Iodine Therapy?
This hypothalamic hormone stimulates the release of ACTH, but its synthetic version is also used to test adrenal reserve.
What is CRH (Corticotropin-Releasing Hormone)?
A 42-year-old female presents with central obesity, a buffalo hump, purple abdominal striae, and proximal muscle weakness. A 1 mg overnight dexamethasone suppression test fails to suppress her morning cortisol.
What is Cushing's Syndrome?
Epinephrine binds to beta-adrenergic receptors, activating G-alpha subunit, which activates adenylyl cyclase. This specific secondary messenger is then produced to activate Protein Kinase A.
What is cAMP (cyclic adenosine monophosphate)?
Produced by adipocytes, this hormone's primary job is to signal the brain that you are full, but its absence causes severe, early-onset morbid obesity.
What is Leptin?
To manage the hirsutism and acne driven by Polycystic Ovary Syndrome (PCOS), clinicians often combine oral contraceptives with this potassium-sparing diuretic due to its off-label ability to act as a competitive androgen receptor antagonist.
What is Spironolactone?
In a patient with primary hypothyroidism, you expect TSH to be elevated. However, if both TSH and Free T4 are low, the failure is located here.
What is the pituitary gland (or hypothalamus / central hypothyroidism)?
A 34-year-old postpartum female presents with severe fatigue, inability to lactate, and loss of pubic hair. She experienced a massive hemorrhage during delivery.
What is Sheehan's Syndrome?
This downstream signaling molecule, which acts as a key energy sensor in the cell, is inhibited by high ATP levels and directly activated by Metformin.
What is AMPK (AMP-activated protein kinase)?
Discovered in the heart, this peptide hormone is released in response to atrial stretch and works to reduce blood volume and lower blood pressure.
What is Atrial Natriuretic Peptide (ANP)?
This class of oral medications lowers postprandial blood sugar by competitively inhibiting an enzyme found on vascular endothelial cells, thereby preventing the rapid degradation of endogenous GLP-1 and GIP.
What are DPP-4 Inhibitors?
This rare autosomal dominant condition mimics primary hyperparathyroidism with high calcium and high/normal PTH, but is differentiated by a very low urinary calcium-to-creatinine clearance ratio (<0.01).
What is Familial Hypocalciuric Hypercalcemia (FHH)?
A 28-year-old female presents with amenorrhea and a prolactin level of 180 ng/mL. An MRI reveals a 4 mm pituitary microadenoma. However, she mentions she was recently started on a medication for chronic nausea. This class of GI drug is the likely culprit.
What is a dopamine antagonist (e.g., Metoclopramide / Reglan)?
Sulfonylureas trigger pancreatic insulin release by directly binding to a specific subunit of an ATP-sensitive channel; this binding changes the channel's conformation to prevent the efflux of this specific intracellular cation.
What is Potassium (K+)?
This hormone, secreted by osteocytes, regulates phosphate homeostasis by downregulating sodium-phosphate co-transporters in the kidney and requires the co-receptor Klotho.
What is FGF23 (Fibroblast Growth Factor 23)?
While mild, asymptomatic cases of SIADH are managed with strict fluid restriction, severe acute hyponatremia causing seizures must be treated carefully with this specific intravenous solution to avoid Central Pontine Myelinolysis.
What is Hypertonic Saline? (or 3% NaCl)