Endocrinology and Hormones

• Endocrinology is the study of hormones, endocrine glands, and related disorders
• Endocrine glands secrete hormones directly into the bloodstream
• Hormones act as chemical messengers, transporting signals throughout the body

Endocrine Glands

• Pituitary gland controls other endocrine glands, and is often called the "master gland"
• Thyroid gland regulates metabolism
• Parathyroid glands control blood calcium levels
• Adrenal glands produce hormones regulating stress response, blood pressure, and electrolyte balance
• Pancreas secretes insulin and glucagon, which regulate blood sugar levels
• Ovaries (in females) produce estrogen and progesterone
• Testes (in males) produce testosterone
• Pineal gland produces melatonin, which regulates sleep

Hormones

• Hormones are chemical substances produced in the body that control and regulate the activity of certain cells or organs
• Hormones are released into the bloodstream and travel to target cells or organs
• They bind to receptors on target cells, triggering a change in the cell's activity

Types of Hormones

• Steroid hormones are lipid-soluble, derived from cholesterol, and include cortisol, estrogen, and testosterone
  • Steroid hormones can pass through the cell membrane and bind to receptors inside the cell
• Peptide hormones are water-soluble, made of amino acids, and include insulin and growth hormone
  • Peptide hormones bind to receptors on the cell surface, triggering a signaling cascade inside the cell
• Amine hormones are modified amino acids, and include epinephrine, norepinephrine, and thyroid hormones
  • Some amine hormones bind to intracellular receptors, and others to cell surface receptors

Hormone Action

• Hormone secretion is typically regulated by negative feedback loops
• A stimulus triggers hormone release
• The hormone causes a response
• The response inhibits further hormone release, maintaining hormone levels within a narrow range
• Hormones exert their effects by binding to receptors on or in target cells
• Receptor binding triggers intracellular signaling pathways, leading to changes in cell function

Hypothalamus and Pituitary

• The hypothalamus and pituitary gland are closely linked and control many endocrine functions
• The hypothalamus secretes releasing and inhibiting hormones that regulate the anterior pituitary
• The posterior pituitary stores and releases hormones produced by the hypothalamus; oxytocin and vasopressin
• The pituitary gland has two lobes: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis)
• The anterior pituitary produces hormones such as growth hormone (GH), prolactin, adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH)

Thyroid Gland

• The thyroid gland produces thyroid hormones (T4 and T3), which regulate metabolism
• T4 is converted to T3, which is the more active form
• Thyroid hormone production is controlled by TSH from the pituitary
• Thyroid disorders include hypothyroidism and hyperthyroidism, which are underactive and overactive thyroid respectively

Adrenal Glands

• The adrenal glands produce hormones in response to stress
• The adrenal cortex (outer layer) produces cortisol, aldosterone, and androgens
  • Cortisol regulates metabolism and stress response
  • Aldosterone regulates sodium and potassium balance
  • Androgens contribute to sexual development and function
• The adrenal medulla (inner layer) produces epinephrine and norepinephrine
  • These hormones mediate the "fight or flight" response

Pancreas

• The pancreas has both endocrine and exocrine functions
• The endocrine part of the pancreas consists of the islets of Langerhans, which contain alpha and beta cells
• Beta cells produce insulin, which lowers blood glucose levels
• Alpha cells produce glucagon, which raises blood glucose levels
• Diabetes mellitus is a common endocrine disorder caused by insulin deficiency or resistance

Calcium Regulation

• Parathyroid hormone (PTH) is produced by the parathyroid glands and increases blood calcium levels
• Calcitonin, produced by the thyroid gland, decreases blood calcium levels
• Vitamin D plays a role in calcium absorption and bone metabolism

Reproductive Hormones

• In females, the ovaries produce estrogen and progesterone, which regulate the menstrual cycle, pregnancy, and female sexual characteristics
• In males, the testes produce testosterone, which regulates sperm production and male sexual characteristics
• FSH and LH, produced by the pituitary gland, control the function of the ovaries and testes

Common Endocrine Disorders

• Diabetes mellitus is characterized by high blood glucose levels due to insulin deficiency or resistance
• Hypothyroidism is an underactive thyroid gland, leading to slowed metabolism
• Hyperthyroidism is an overactive thyroid gland, leading to increased metabolism
• Cushing's syndrome is excessive cortisol production
• Addison's disease is adrenal insufficiency, leading to decreased cortisol and aldosterone production
• Growth disorders are caused by GH deficiency or excess
• Polycystic ovary syndrome (PCOS) is a hormonal disorder in women, leading to irregular periods, cysts on the ovaries, and increased androgen levels

Endocrine Diagnostic Tests

• Blood tests measure hormone levels
• Urine tests measure hormone levels
• Stimulation tests assess the response of an endocrine gland to stimulation
• Suppression tests assess the ability of an endocrine gland to be suppressed
• Imaging studies such as CT scans, MRI, and ultrasound visualize endocrine glands

Treatment of Endocrine Disorders

• Hormone replacement therapy treats hormone deficiencies
• Medications suppress hormone production or block hormone action
• Surgery removes tumors or abnormal endocrine tissue
• Lifestyle modifications, such as diet and exercise manage diabetes

The Pineal Gland

• The pineal gland secretes melatonin
• Melatonin helps regulate sleep-wake cycles (circadian rhythms)
• Melatonin secretion is influenced by light exposure

Prostaglandins

• Prostaglandins are hormone-like substances derived from fatty acids
• Prostaglandins act locally via paracrine signaling
• Prostaglandins are involved in inflammation, pain, and fever

Endocrine Disruptors

• Endocrine disruptors are chemicals that interfere with the endocrine system
• They can mimic or block hormone action
• Examples include certain pesticides, plastics, and industrial chemicals
• Endocrine disruptors can have adverse effects on development, reproduction, and immune function