Endocrine System — Set 3

Correct Answer: C. Prolactin

• **Prolactin** = Prolactin (PRL) is secreted by the anterior pituitary after childbirth; high prolactin levels activate alveolar cells in the mammary glands to synthesise and secrete breast milk (lactogenesis), enabling breastfeeding. • **Key fact** — During pregnancy, rising estrogen and progesterone suppress prolactin's milk-producing action despite high prolactin levels; only after delivery, when these hormones drop sharply, does prolactin trigger actual milk production. • Elevated prolactin outside pregnancy (hyperprolactinemia) can suppress ovulation and cause infertility, often due to a benign pituitary tumour called a prolactinoma. • 💡 Option A (Progesterone) is wrong because it prepares the uterus for implantation and maintains pregnancy, not milk synthesis; Option B (Estrogen) is wrong because it promotes breast tissue growth during pregnancy but suppresses lactation; Option D (Oxytocin) is wrong because it triggers the milk ejection (let-down) reflex once milk is already present, but does not synthesise it.

Correct Answer: A. Adrenal Cortex

• **Adrenal Cortex** = The adrenal cortex is the outer layer of the adrenal gland and is divided into three zones — the zona glomerulosa (outermost) specifically produces aldosterone, a mineralocorticoid that tells the kidney's tubules to retain sodium and excrete potassium, directly controlling blood volume and pressure. • **Key fact** — Aldosterone secretion is triggered by angiotensin II and high blood potassium, forming part of the RAAS; excess aldosterone causes Conn's syndrome (hypertension + low potassium). • The adrenal medulla (inner zone) is entirely separate, secreting adrenaline and noradrenaline — not aldosterone. • 💡 Option B (Kidneys) is wrong because the kidneys respond to aldosterone but do not produce it; Option C (Thyroid) is wrong because it produces thyroxine and calcitonin; Option D (Adrenal Medulla) is wrong because the medulla produces catecholamines (adrenaline, noradrenaline), not aldosterone.

Correct Answer: C. Thyroid

• **Thyroid** = Calcitonin is produced by the parafollicular C-cells (not the follicular cells) of the thyroid gland; it lowers blood calcium by inhibiting osteoclast activity (bone breakdown) and enhancing calcium excretion by the kidneys. • **Key fact** — Calcitonin acts as the antagonist to Parathyroid Hormone (PTH) — while PTH raises blood calcium, calcitonin lowers it; calcitonin is sometimes used medically to treat osteoporosis and hypercalcemia. • The word 'calcitonin' derives from 'calci' (calcium) + 'tonin' (tone/regulate), reflecting its role in maintaining calcium tone. • 💡 Option A (Thymus) is wrong because it produces thymosin for T-lymphocyte development; Option B (Parathyroid) is wrong because the parathyroid produces PTH, which raises blood calcium — the opposite effect; Option D (Pancreas) is wrong because the pancreas produces insulin and glucagon for blood sugar regulation.

Correct Answer: C. Thyroid Stimulating Hormone

• **Thyroid Stimulating Hormone** = TSH (thyrotropin) is secreted by the anterior pituitary in response to TRH from the hypothalamus; it binds to receptors on thyroid follicular cells and stimulates every step of thyroid hormone synthesis and release — from iodine uptake to T3/T4 secretion. • **Key fact** — In primary hypothyroidism (underactive thyroid), T3/T4 levels fall, so TSH rises dramatically in a negative feedback attempt; blood TSH is therefore the first test doctors order to screen for thyroid disorders. • Autoantibodies that mimic TSH (TSI) are the cause of Graves' disease, stimulating the thyroid without pituitary control. • 💡 Option A (Follicle Stimulating Hormone) is wrong because FSH stimulates egg follicle growth in females and sperm production in males; Option B (Luteinizing Hormone) is wrong because LH triggers ovulation and testosterone secretion; Option D (Adrenocorticotropic Hormone/ACTH) is wrong because ACTH stimulates the adrenal cortex to produce cortisol, not the thyroid.

Correct Answer: D. Adrenal

• **Adrenal** = Addison's disease (primary adrenal insufficiency) occurs when the adrenal cortex is destroyed (often by autoimmune attack) and fails to produce sufficient cortisol and aldosterone, causing fatigue, weight loss, low blood pressure, darkened skin, and life-threatening salt wasting. • **Key fact** — The bronze skin darkening characteristic of Addison's disease happens because the pituitary over-produces ACTH in compensation, and ACTH shares a precursor (POMC) with MSH (melanocyte-stimulating hormone), which darkens the skin. • President John F. Kennedy had Addison's disease, which was kept secret during his presidency. • 💡 Option A (Pancreas) is wrong because its insufficiency causes diabetes, not Addison's symptoms; Option B (Thyroid) is wrong because its underactivity causes hypothyroidism; Option C (Pituitary) is wrong because pituitary underactivity causes secondary adrenal insufficiency (no skin darkening) and is a different diagnosis.

Correct Answer: A. Somatotropin

• **Somatotropin** = Somatotropin (Growth Hormone, GH) is secreted by somatotroph cells of the anterior pituitary; it stimulates the liver to release IGF-1, which directly promotes division of chondrocytes in growth plates of long bones and increases protein synthesis in all body tissues. • **Key fact** — Excess somatotropin during childhood causes gigantism (abnormally tall stature), while excess in adults (after growth plates close) causes acromegaly with enlarged hands, feet, jaw, and internal organs. • Somatotropin secretion peaks during deep sleep and exercise, which is why adequate sleep is critical for growth in children. • 💡 Option B (Thyroxine) is wrong because it regulates metabolic rate and overall development but does not directly drive bone length; Option C (Testosterone) is wrong because it promotes muscle growth and secondary sexual development, not skeletal elongation; Option D (Insulin) is wrong because it regulates blood glucose, though it has mild anabolic effects.

Correct Answer: A. Thyroid

• **Thyroid** = The thyroid is a butterfly-shaped gland situated in the anterior neck, wrapping around the trachea just below the thyroid cartilage (Adam's apple); its two lobes are connected by a narrow band called the isthmus. • **Key fact** — The thyroid is the largest purely endocrine gland in the body; it stores up to 3 months' worth of thyroid hormones in colloid-filled follicles, making it unique among endocrine glands. • Four small parathyroid glands are embedded on the posterior surface of the thyroid but are functionally separate. • 💡 Option B (Pituitary) is wrong because the pituitary is a pea-sized gland at the base of the brain, far from the Adam's apple; Option C (Adrenal) is wrong because the adrenal glands sit atop the kidneys in the abdomen; Option D (Pineal) is wrong because the pineal gland is a tiny structure in the centre of the brain.

Correct Answer: D. Erythropoietin

• **Erythropoietin** = Erythropoietin (EPO) is secreted by peritubular cells of the kidney in response to low oxygen (hypoxia); it travels to the bone marrow and stimulates stem cells to differentiate into red blood cells, increasing the oxygen-carrying capacity of blood. • **Key fact** — EPO is abused in endurance sports (blood doping) because more red blood cells deliver more oxygen to muscles; synthetic recombinant EPO (rEPO) is also used to treat anaemia in chronic kidney disease and chemotherapy patients. • The kidneys continuously monitor blood oxygen via special oxygen-sensing cells and adjust EPO output accordingly. • 💡 Option A (Calcitriol) is wrong because calcitriol is the active form of Vitamin D, produced by the kidneys to regulate calcium absorption; Option B (Thymosin) is wrong because it is produced by the thymus for T-cell maturation; Option C (Renin) is wrong because it is an enzyme secreted by the kidneys that activates the RAAS to regulate blood pressure, not red cell production.

Correct Answer: B. Raises blood sugar

• **Raises blood sugar** = Glucagon is secreted by alpha cells of the pancreatic Islets of Langerhans when blood glucose falls too low; it stimulates glycogenolysis (breaking down liver glycogen into glucose) and gluconeogenesis (making new glucose from amino acids), thus raising blood glucose. • **Key fact** — Glucagon and insulin are antagonists: insulin lowers blood sugar (secreted after eating) while glucagon raises it (secreted during fasting); this opposing pair keeps blood glucose within a narrow range of ~70–100 mg/dL. • Injectable glucagon kits are used to treat severe hypoglycemia in diabetic patients who are unconscious and cannot consume sugar orally. • 💡 Option A (Regulates sleep) is wrong because sleep regulation is the function of melatonin from the pineal gland; Option C (Promotes milk production) is wrong because that is prolactin's role; Option D (Lowers blood sugar) is wrong because that is the function of insulin, the opposite hormone.

Correct Answer: D. Pancreas

• **Pancreas** = The pancreas is a dual-purpose (heterocrine) organ — its exocrine portion (acinar cells) secretes digestive enzymes (amylase, lipase, protease) into the small intestine via the pancreatic duct, while its endocrine portion (Islets of Langerhans) secretes insulin, glucagon, and somatostatin directly into the bloodstream. • **Key fact** — About 99% of the pancreatic tissue is exocrine; only 1–2% consists of the Islets of Langerhans, yet these tiny clusters have enormous metabolic importance. • The pancreas is located behind the stomach in the retroperitoneal space and is approximately 15 cm long. • 💡 Option A (Pituitary) is wrong because it is purely an endocrine gland with no digestive function; Option B (Adrenal) is wrong because it is also purely endocrine, secreting cortisol, adrenaline, and aldosterone; Option C (Thyroid) is wrong because it secretes thyroid hormones and calcitonin but has no exocrine role.