Key Points

Endocrine glands are ductless glands that release secretions called hormones directly into the bloodstream. Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts.

The hypothalamus produces releasing hormones (like Gonadotrophin releasing hormone, GnRH) and inhibiting hormones (like somatostatin) that regulate the synthesis and secretion of pituitary hormones.

The anterior pituitary produces Growth Hormone (GH), Prolactin (PRL), Thyroid Stimulating Hormone (TSH), Adrenocorticotrophic Hormone (ACTH), Luteinizing Hormone (LH), and Follicle Stimulating Hormone (FSH).

The posterior pituitary stores and releases oxytocin and vasopressin (Antidiuretic Hormone, ADH), which are synthesized by the hypothalamus. Oxytocin aids in childbirth and milk ejection, while ADH regulates water reabsorption by the kidney.

The pineal gland, located on the dorsal side of the forebrain, secretes melatonin. Melatonin regulates the 24-hour (diurnal) rhythm of our body, such as the sleep-wake cycle.

The thyroid gland secretes thyroxine ($T_4$) and triiodothyronine ($T_3$), which regulate the basal metabolic rate. It also secretes thyrocalcitonin (TCT), a protein hormone that regulates blood calcium levels.

The parathyroid glands secrete parathyroid hormone (PTH), a hypercalcemic hormone. PTH increases blood calcium ($Ca^{2+}$) levels by stimulating bone resorption and calcium reabsorption in the kidneys.

The thymus gland secretes peptide hormones called thymosins, which play a major role in the differentiation of T-lymphocytes for cell-mediated immunity and promote antibody production for humoral immunity.

The adrenal medulla secretes adrenaline (epinephrine) and noradrenaline (norepinephrine), known as catecholamines. These are emergency hormones for 'fight or flight' responses during stress.

The adrenal cortex secretes corticoids. Glucocorticoids (e.g., cortisol) manage carbohydrate metabolism, while mineralocorticoids (e.g., aldosterone) regulate water and electrolyte balance.

The Islets of Langerhans in the pancreas contain $\alpha$-cells that secrete glucagon (a hyperglycemic hormone) and $\beta$-cells that secrete insulin (a hypoglycemic hormone) to maintain blood glucose homeostasis.

The testis functions as an endocrine gland, producing a group of hormones called androgens, mainly testosterone. Androgens regulate male sexual development, spermatogenesis, and secondary sexual characteristics.

The ovary produces two groups of steroid hormones: estrogen and progesterone. Estrogen is responsible for female secondary sexual characteristics, while progesterone supports pregnancy.

The heart's atrial wall secretes atrial natriuretic factor (ANF) to decrease blood pressure. The kidneys produce erythropoietin to stimulate red blood cell (RBC) formation.

The GI tract secretes four major peptide hormones: gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP), which regulate digestion.

Peptide hormones bind to specific receptors on the cell membrane and generate second messengers like cyclic AMP (cAMP) or $Ca^{2+}$ to trigger a cellular response without entering the cell.

Steroid and thyroid hormones are lipid-soluble, so they enter the target cell and bind to intracellular receptors. This hormone-receptor complex interacts with the genome to regulate gene expression.