Other Endocrine Glands

Pineal Gland

The pineal gland is a small endocrine organ 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 PMS; decreased by phototherapy.

• Effects: Depression, sleepiness, irritability, and carbohydrate craving.

Thymus

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

• Function: Secretes hormones that regulate the development of T-cells (immune cells).

Parathyroid Glands

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

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

Pancreas

The pancreas is a retroperitoneal organ, inferior and dorsal to the stomach. It has both endocrine and exocrine functions.

• Islets (2%): Produce hormones (endocrine).

• 98%: Produces digestive enzymes (exocrine).

Pancreatic Hormones

Insulin (from β cells)

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

• Stimulates glucose and amino acid uptake.

• Antagonizes glucagon.

Glucagon (from α cells)

Glucagon is secreted during fasting or very low carbohydrate diets.

• Stimulates breakdown of glycogen and fat catabolism.

• Antagonizes insulin.

Diabetes Mellitus

Signs and Symptoms

Diabetes mellitus is characterized by hyposecretion or resistance to insulin.

• Polyuria (excessive urination), polydipsia (excessive thirst), polyphagia (excessive hunger).

• Hyperglycemia, glycosuria, ketonuria.

• Osmotic diuresis: Increased glucose draws water into urine by osmosis.

Type I Diabetes Mellitus

Type I diabetes is an autoimmune disorder resulting in destruction of β cells, usually diagnosed in children.

• 10% of diabetes cases.

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

Type II Diabetes Mellitus

Type II diabetes is caused by insulin resistance, where target cells fail to respond to insulin.

• 90% of diabetes cases.

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

• Treatment: Weight loss, diet, exercise, oral medications.

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 inhibiting glucagon release.

• Decrease appetite, inhibit gastric emptying.

• Induce insulin secretion, inhibit glucagon secretion.

Hyperinsulinism

Hyperinsulinism results from excess insulin injection or pancreatic islet tumor.

• Causes hypoglycemia, weakness, hunger.

• Triggers secretion of epinephrine, GH, glucagon.

• Side effects: Anxiety, sweating, increased heart rate.

• Insulin shock: Disorientation, convulsions, unconsciousness (diabetic coma).

Gestational Diabetes

Gestational diabetes occurs during pregnancy due to placental hormones deactivating insulin receptors.

• Reverses after birth.

• Can cause birth defects and maternal health problems.

• Risk factor for development of type II diabetes in mother.

Gonads

Ovaries

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

Testes

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

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), continues calcitriol synthesis.

• Kidneys: Produce 85% of 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 (water-soluble): Histamine, epinephrine, insulin.

• Steroid hormones (lipid-soluble): Derived from cholesterol.

• Hormones act only on target 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, binds to DNA, and initiates transcription of mRNA, leading to protein synthesis.

• Example: Steroid hormones (e.g., estrogen, testosterone).

Water-Soluble Hormone Mechanism of Action

Water-soluble hormones bind to cell-surface receptors and activate second messenger systems, such as cAMP, to trigger cellular responses.

• Example: Epinephrine, insulin.

Enzyme Amplification

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

• Hormone activates cAMP and protein kinase.

• Activated enzymes produce metabolic products.

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 cells can adjust their sensitivity to hormones by up-regulating or down-regulating receptor density.

• Up-regulation: Increased receptor density, stronger response.

• Down-regulation: Reduced receptor density, 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.

Stress and Adaptation

Stress

Stress is caused by any situation that disrupts homeostasis and threatens physical or emotional well-being.

• Examples: Exercise, pregnancy, illness, starvation, sleep deprivation, emotional trauma.

General Adaptation Syndrome

The body reacts 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 or transported in blood.

• 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 (Aspirin, Advil).

• COX-2 inhibitors: Vioxx, Celebrex.

• Steroidal anti-inflammatory: Prednisone.