Other Endocrine Glands

Pineal Gland

The pineal gland is a small endocrine gland located in the brain, responsible for regulating circadian rhythms and possibly the timing of puberty. It produces serotonin during the day and converts it to melatonin at night.

• Peak secretion: Ages 1-5; decreases by 75% at puberty.

• Melatonin: Regulates sleep-wake cycles; increased in Seasonal Affective Disorder (SAD) and Premenstrual Syndrome (PMS); decreased by phototherapy.

• Symptoms of melatonin imbalance: Depression, sleepiness, irritability, carbohydrate craving.

Thymus

The thymus is located in the mediastinum, superior to the heart. It is essential for immune function, especially during childhood.

• Function: Secretes hormones that regulate the development of T-cells (critical for adaptive immunity).

• Involution: Shrinks after puberty.

Parathyroid Glands

The parathyroid glands are small glands located on the posterior aspect of the thyroid gland. They regulate calcium homeostasis.

• PTH (Parathyroid Hormone): Released in response to low blood Ca2+ levels.

• Effect: Increases Ca2+ in blood by stimulating bone resorption, increasing intestinal absorption, and reducing renal excretion.

Pancreas

The pancreas is a retroperitoneal organ with both endocrine and exocrine functions. Its islets produce hormones, while the majority of the organ produces digestive enzymes.

• Islets of Langerhans: Cell clusters that produce hormones (2% of pancreatic tissue).

• Exocrine function: 98% of the pancreas produces digestive enzymes.

Pancreatic Hormones

Insulin (from β cells)

Insulin is secreted after meals when carbohydrate and/or protein intake raises blood glucose levels.

• Function: Stimulates glucose and amino acid uptake by cells; antagonizes glucagon.

Glucagon (from α cells)

Glucagon is secreted during fasting or very low carbohydrate diets.

• Function: Stimulates breakdown of glycogen and fat catabolism; antagonizes insulin.

Diabetes Mellitus

Signs and Symptoms

Diabetes mellitus is characterized by hyposecretion or resistance to insulin.

• Symptoms: Polyuria (excessive urination), polydipsia (excessive thirst), polyphagia (excessive hunger), hyperglycemia, glycosuria, ketonuria, osmotic diuresis.

• Pathophysiology: Increased blood glucose leads to glucose in urine; increased glucose draws water into urine by osmosis.

Type I Diabetes Mellitus

Type I Diabetes Mellitus (Insulin Dependent Diabetes Mellitus, IDDM) accounts for 10% of cases and is caused by autoimmune destruction of β cells.

• Onset: Usually diagnosed around age 12.

• Treatment: Diet, exercise, blood glucose monitoring, periodic insulin injections.

Type II Diabetes Mellitus

Type II Diabetes Mellitus (Non-Insulin Dependent Diabetes Mellitus, NIDDM) represents 90% of cases and is due to insulin resistance.

• Risk factors: Heredity, age (40+), obesity.

• Treatment: Weight loss, diet, exercise, oral medications to improve insulin secretion or sensitivity.

Long-Term Consequences

• Vascular damage: Heart and kidney disease, retinal damage (blindness), poor wound healing (limb amputations).

Semaglutides and GLP-1 Receptor Agonists

Semaglutides are GLP-1 receptor agonists used in diabetes treatment. They mimic the action of glucagon-like peptide-1, enhancing insulin secretion and reducing glucagon secretion.

• Mechanism: Food triggers GLP-1 release, which decreases appetite, induces insulin secretion, and inhibits glucagon secretion.

Hyperinsulinism

Hyperinsulinism results from excess insulin injection or pancreatic islet tumor.

• Symptoms: Hypoglycemia, weakness, hunger, anxiety, sweating, increased heart rate.

• Insulin shock: Severe hypoglycemia leading to disorientation, convulsions, or unconsciousness (diabetic coma).

Gestational Diabetes

Gestational diabetes occurs during pregnancy due to placental hormones deactivating insulin receptors. It usually reverses after birth but can cause birth defects and maternal health problems.

• Risk: Increased risk for development of Type II diabetes in the mother.

Gonads

Ovaries

The ovaries secrete estrogens and progesterone, which regulate female reproductive system development, menstrual cycle, pregnancy, and mammary gland preparation for lactation.

• Estrogens: Promote bone growth and female physique.

• Progesterone: Sustains pregnancy and prepares mammary glands.

Testes

The testes produce androgens, which regulate male reproductive system development, sustain sperm production, and sex drive.

• Androgens: Promote male physique and reproductive function.

Endocrine Functions of Other Organs

• Heart: Releases atrial natriuretic peptide (ANP) to decrease blood volume and blood pressure.

• Stomach and small intestines: Produce 10 enteric hormones to coordinate digestive motility and secretion.

• Liver: Produces erythropoietin (stimulates RBC production) and continues calcitriol synthesis.

• Kidneys: Produce most erythropoietin.

• Placenta: Secretes estrogen, progesterone, and other hormones to regulate pregnancy and fetal development.

Hormone Physiology

Chemical Nature of Hormones

Hormones are classified by their chemical nature, which determines their solubility and mechanism of action.

• Protein-based hormones: Water-soluble (e.g., histamine, epinephrine, insulin).

• Steroid hormones: Lipid-soluble.

• Target cells: Hormones act only on cells with specific receptors.

Hormone Mode of Action

• Lipid-soluble hormones: Penetrate plasma membrane and enter nucleus.

• Water-soluble hormones: Bind to cell-surface receptors.

Lipid-Soluble Hormone Mode of Action

Lipid-soluble hormones, such as steroids, diffuse through the plasma membrane and bind to intracellular receptors. The receptor-hormone complex enters the nucleus and binds to DNA, initiating transcription and protein synthesis.

• Steps: Diffusion → receptor binding → nuclear entry → DNA binding → mRNA synthesis → protein synthesis.

Water-Soluble Hormone Mechanism of Action

Water-soluble hormones bind to cell-surface receptors, activating second messenger systems (e.g., cAMP) to trigger cellular responses.

• Second messenger: cAMP, produced by adenylate cyclase, activates protein kinases.

Enzyme Amplification

Enzyme amplification allows a small stimulus (hormone) to produce a large effect through a cascade of reactions.

• Mechanism: Hormone → cAMP/protein kinase → activated enzymes → metabolic product.

Hormone Clearance

Hormone signals must be terminated after their action. Hormones are taken up and degraded by the liver and kidneys, then excreted in bile or urine.

• Metabolic clearance rate (MCR): Rate of hormone removal.

• Half-life: Time required to clear 50% of hormone.

Modulation of Target Cell Sensitivity

Target cell sensitivity to hormones can be modulated by up-regulation (increased receptor density) or down-regulation (decreased receptor density).

• Up-regulation: Increased sensitivity and stronger response.

• Down-regulation: Reduced sensitivity and diminished response.

Control of Pituitary: Feedback from Target Organs

The pituitary gland is regulated by feedback from target organs, often through negative feedback mechanisms.

• Example: Thyroid hormone inhibits release of TRH and TSH by negative feedback.

Stress and Adaptation

Stress

Stress is caused by any situation that disrupts homeostasis and threatens physical or emotional well-being. Examples include exercise, pregnancy, illness, starvation, sleep deprivation, divorce, death, or accidents.

General Adaptation Syndrome

The General Adaptation Syndrome describes the body's response to stress in three stages: alarm reaction, resistance, and exhaustion.

• Alarm reaction: Mobilizes resources.

• Resistance: Copes with stressor.

• Exhaustion: Reserves depleted.

Paracrine Secretions

Paracrine secretions are chemical messengers that diffuse short distances and stimulate nearby cells. They are not produced in neurons (unlike neurotransmitters) and are not transported in blood (unlike hormones).

• Histamine: Causes vasodilation; involved in allergies.

• Nitric oxide: Causes vasodilation from blood vessels.

• Eicosanoids: Diverse functions; involved in inflammation, pain, fever.

Eicosanoids: Paracrine Secretions

Eicosanoids mediate allergic and inflammatory reactions, stimulate vasoconstriction and clotting, mediate smooth muscle contraction and relaxation, and sensitize neurons to pain.

• Non-steroidal anti-inflammatory drugs (NSAIDs): COX-1/2 inhibitors (e.g., Aspirin, Advil).

• COX-2 inhibitors: Vioxx, Celebrex.

• Steroidal anti-inflammatory drugs: Prednisone.