Other Endocrine Glands

Pineal Gland

The pineal gland is a small endocrine gland located in the brain. It is most active in early childhood, with peak secretion between ages 1-5, and its activity decreases significantly by puberty. The pineal gland produces serotonin during the day and converts it to melatonin at night, which regulates the circadian rhythm (sleep-wake cycle). Melatonin may also influence the timing of puberty in humans. Elevated melatonin levels are associated with Seasonal Affective Disorder (SAD) and Premenstrual Syndrome (PMS), and phototherapy can reduce melatonin to alleviate symptoms such as depression, sleepiness, irritability, and carbohydrate craving.

Thymus

The thymus is located in the mediastinum, superior to the heart. It is prominent in childhood and shrinks after puberty. The thymus secretes hormones that regulate the development of T-cells, which are essential for the immune response.

Parathyroid Glands

The parathyroid glands are small glands located on the posterior aspect of the thyroid gland. They secrete parathyroid hormone (PTH) in response to low blood calcium levels, which increases calcium concentration in the blood by stimulating bone resorption, increasing intestinal absorption, and reducing urinary excretion of calcium.

Pancreas

The pancreas is a retroperitoneal organ, located inferior and dorsal to the stomach. It has both endocrine and exocrine functions. The endocrine portion consists of islets (cell clusters) that produce hormones (2% of the organ), while the remaining 98% produces digestive enzymes. The main pancreatic hormones are insulin (from β cells) and glucagon (from α cells).

• Insulin: Secreted after meals when carbohydrate and/or protein intake raises blood glucose levels. It stimulates glucose and amino acid uptake and antagonizes glucagon.

• Glucagon: Secreted during fasting or very low carbohydrate diets. It stimulates the breakdown of glycogen and fat catabolism, antagonizing insulin.

Diabetes Mellitus

Signs and Symptoms

Diabetes mellitus is characterized by hyposecretion or resistance to insulin. Common symptoms include polyuria (excessive urination), polydipsia (excessive thirst), polyphagia (excessive hunger), hyperglycemia (high blood glucose), glycosuria (glucose in urine), and ketonuria (ketones in urine). Osmotic diuresis occurs as increased glucose draws water into urine by osmosis.

Type I Diabetes Mellitus

Type I diabetes (Insulin Dependent Diabetes Mellitus, IDDM) accounts for 10% of cases. It is caused by autoimmune destruction of β cells and is usually diagnosed around age 12. Treatment includes diet, exercise, blood glucose monitoring, and periodic insulin injections.

Type II Diabetes Mellitus

Type II diabetes (Non-Insulin Dependent Diabetes Mellitus, NIDDM) represents 90% of cases and is due to insulin resistance, where target cells fail to respond to insulin. Major risk factors include heredity, age (40+), and obesity. Treatment involves weight loss, diet, exercise, and oral medications to improve insulin secretion or sensitivity.

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, and increases the risk of developing type II diabetes in the mother.

Gonads and Endocrine Functions of Other Organs

Gonads

The ovaries secrete estrogens and progesterone, which regulate the development of the female reproductive system, menstrual cycle, pregnancy, and prepare mammary glands for lactation. The testes produce androgens, which regulate the development of the male reproductive system, sustain sperm production, and sex drive.

Other Organs

• Heart: Releases atrial natriuretic peptide (ANP) to decrease blood volume and pressure; produces erythropoietin to stimulate RBC production.

• Liver: Continues calcitriol synthesis and produces some erythropoietin.

• Stomach and Small Intestines: Produce enteric hormones to coordinate digestive motility and secretion.

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

• Kidneys: Produce hormones involved in blood pressure regulation and erythropoiesis.

Hormone Physiology

Chemical Nature of Hormones

Most hormones are protein-based (water-soluble), such as histamine, epinephrine, and insulin. Steroid hormones are lipid-soluble. Hormones circulate throughout the body but only act on target cells with specific receptors. There are two main mechanisms of hormone action: lipid-soluble and water-soluble.

Hormone Mode of Action

• Lipid-soluble hormones: Penetrate the plasma membrane and enter the nucleus to affect gene expression.

• Water-soluble hormones: Bind to cell-surface receptors and activate second messenger systems.

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 to mRNA, leading to protein synthesis.

Water-Soluble Hormone Mechanism of Action

Water-soluble hormones bind to cell-surface receptors, activating second messenger systems such as cAMP. This leads to enzyme amplification and a cascade of cellular responses.

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. The metabolic clearance rate (MCR) is the rate of hormone removal, and the half-life is the time required to clear 50% of the hormone.

Modulation of Target Cell Sensitivity

Target cells can modulate their sensitivity to hormones by up-regulating (increasing receptor density) or down-regulating (decreasing receptor density) their receptors, affecting the strength of the response.

Control of Pituitary: Feedback from Target Organs

The pituitary gland is regulated by feedback from target organs. Hormones released by the pituitary stimulate target organs, which in turn send feedback signals to modulate pituitary activity.

Stress and Adaptation

General Adaptation Syndrome

Stress is caused by any situation that disrupts homeostasis and threatens physical or emotional well-being. The body reacts to stress in three stages, known as the General Adaptation Syndrome: alarm reaction, resistance, and exhaustion.

Paracrine Secretions

Paracrine secretions are chemical messengers that diffuse short distances and stimulate nearby cells. Unlike neurotransmitters, they are not produced in neurons, and unlike hormones, they are not transported in blood. Examples include histamine (causes vasodilation in allergies), nitric oxide (causes vasodilation), and eicosanoids (involved in inflammation, pain, fever).