The endocrine system regulates body functions via hormones from ductless glands. These glands include the hypothalamus, pituitary, thyroid, parathyroid, adrenals, pancreas, gonads, pineal gland, etc. These glands send signals over long distances by releasing secretions directly into the blood. These signals help control growth, metabolism, and homeostasis. The hypothalamus controls anterior pituitary tropic hormones (which affect the thyroid, adrenal glands, and gonads) and stores posterior hormones like oxytocin and vasopressin. The suprachiasmatic nucleus sets the body’s circadian rhythms for cortisol and melatonin. Hormones have strong, amplified effects at picomolar–nanomolar levels through specific receptors. The activation of receptors starts cascades or changes gene expression. Signaling can be endocrine (distant), paracrine (local), autocrine (self), or intracrine (inside the cell). Pulsatile release keeps sensitivity high, and receptor expression makes sure that tissues respond in different ways. Peptides and proteins like thyrotropin-releasing hormone, insulin, and growth hormone use cell-surface receptors called G protein-coupled receptors and receptor tyrosine kinases. Adrenal and gonadal steroids like cortisol and testosterone bind to nuclear receptors to change genes. Catecholamines (epinephrine) come from amino acids through G protein-coupled receptors. Eicosanoids and vitamin D add paracrine and nuclear modes. Negative feedback keeps levels steady. Positive loops regulate ovulation and parturition. Cells can directly control things like glucose and calcium by sensing them. Neural and immune integration makes this more accurate.