Chapter 6: Endocrine System — Hormones, Feedback, and Diabetes

The endocrine system uses chemical messengers (hormones) secreted into the bloodstream to regulate virtually every physiological process: metabolism, growth, reproduction, stress response, and fluid balance.

Major Endocrine Glands and Hormones

Gland	Key Hormones	Primary Actions
Hypothalamus	TRH, CRH, GnRH, GHRH, somatostatin	Controls pituitary hormone release
Anterior pituitary	TSH, ACTH, LH, FSH, GH, Prolactin	Stimulates other endocrine glands
Posterior pituitary	ADH (vasopressin), Oxytocin	Water retention; uterine contraction/bonding
Thyroid	T₃, T₄, Calcitonin	Metabolic rate, growth, heart rate, Ca²⁺ lowering
Parathyroid	PTH	↑ Blood Ca²⁺ (↑ bone resorption, ↑ renal reabsorption, ↑ calcitriol)
Adrenal cortex	Cortisol, Aldosterone, DHEA	Stress response, Na⁺ retention, androgens
Adrenal medulla	Epinephrine, Norepinephrine	Acute stress (fight or flight)
Pancreas (islets)	Insulin (β cells), Glucagon (α cells)	Blood glucose regulation
Gonads	Estrogen, Progesterone, Testosterone	Reproduction, secondary sex characteristics

Negative Feedback Mechanism

Most hormonal axes use negative feedback to maintain homeostasis:

HPT axis example: Hypothalamus → TRH → Anterior pituitary → TSH → Thyroid gland → T₃/T₄

Rising T₃/T₄ feeds back to inhibit TRH and TSH release, preventing excess production. This is the principle behind diagnosing thyroid disorders: high TSH + low T₄ = primary hypothyroidism; low TSH + high T₄ = hyperthyroidism (or secondary hypothyroidism if TSH is also low).

Diabetes Mellitus

Type 1 DM: Autoimmune destruction of pancreatic β cells → absolute insulin deficiency. Presents with DKA (diabetic ketoacidosis): hyperglycemia + ketonemia + metabolic acidosis. Treatment: insulin replacement (basal-bolus regimen).

Type 2 DM: Progressive insulin resistance + relative insulin deficiency. Strongly associated with obesity, physical inactivity, and genetic predisposition. Treatment: lifestyle modification → metformin (first-line) → add GLP-1 agonists, SGLT-2 inhibitors, sulfonylureas, or insulin as needed.

Key monitoring parameters:

• HbA1c: reflects average blood glucose over 2–3 months; target <7% for most adults
• Fasting glucose: target 80–130 mg/dL
• Postprandial glucose: target <180 mg/dL (2h post-meal)

Chronic complications (mnemonic: NRCP):

• Neuropathy (peripheral, autonomic)
• Retinopathy (leading cause of blindness in working-age adults)
• Cardiovascular disease (2–4× increased risk)
• Nephropathy → CKD → end-stage renal disease

Thyroid Disorders

Condition	TSH	Free T₄	Symptoms
Primary hypothyroidism	↑	↓	Fatigue, cold intolerance, weight gain, constipation
Hyperthyroidism (Graves’)	↓	↑	Palpitations, heat intolerance, weight loss, exophthalmos
Subclinical hypothyroidism	↑	Normal	Often asymptomatic

Key Checklist

• Lists the major endocrine glands and their principal hormones with functions
• Explains the HPT axis negative feedback loop and applies it to diagnosing thyroid disease
• Distinguishes Type 1 from Type 2 DM by pathophysiology, presentation, and treatment