🧬 Chemical Coordination & Endocrine Signaling

Endocrine glands are ductless glands that secrete hormones directly into the bloodstream to modulate physiological target cells via specific high-affinity receptors.

1. 🏛️ Master Endocrine Glands & Hormone Profiles

                          HYPOTHALAMIC-PITUITARY AXIS
                                 HYPOTHALAMUS
                         (Releasing / Inhibiting Hormones)
                                      │
                                      ▼ [Hypophyseal Portal System]
                             ANTERIOR PITUITARY (Adenohypophysis)
             ┌────────────────────────┼────────────────────────┐
             ▼                        ▼                        ▼
      GH, PRL, TSH, ACTH           LH, FSH (Gonadotropins)    MSH (Pars Intermedia)
                                      │
                                      ▼ [Direct Axonal Transport via Neurohypophysis]
                             POSTERIOR PITUITARY: Oxytocin & Vasopressin (ADH)

Endocrine Gland	Key Hormones Secreted	Target Tissue & Physiological Actions	Clinical Endocrine Pathology
Anterior Pituitary	Growth Hormone (GH)	Epiphyseal bone plates, liver (IGF-1 synthesis)	Gigantism / Acromegaly (Hyper); Pituitary Dwarfism (Hypo).
Posterior Pituitary	Oxytocin & ADH (Vasopressin)	Uterus (myometrial labor contractions); Renal collecting duct ( $H_{2} O$ reabsorption via aquaporins)	Diabetes Insipidus (ADH deficiency $⟹$ severe polyuria & polydipsia).
Thyroid Gland	$T_{3}$ (Triiodothyronine), $T_{4}$ (Thyroxine), Calcitonin	Basal Metabolic Rate (BMR), thermogenesis, erythropoiesis; Calcitonin lowers blood $C a^{2 +}$	Cretinism (Congenital hypo); Grave's Disease / Exophthalmic Goitre (Hyper); Simple Goitre ( $I_{2}$ deficiency).
Parathyroid Gland	Parathyroid Hormone (PTH / Collip's)	Bone osteoclasts, kidney tubules, gut absorption	Hypercalcemic hormone: Elevates blood $C a^{2 +}$ by bone demineralization (Antagonistic to Calcitonin).
Adrenal Cortex	Glucocorticoids (Cortisol), Mineralocorticoids (Aldosterone)	Liver gluconeogenesis, anti-inflammatory, immunosuppression; Renal $N a^{+} / H_{2} O$ retention	Cushing Syndrome (Hypercortisolism); Addison's Disease (Hypo-adrenocorticism: bronze pigmentation).
Adrenal Medulla	Adrenaline (Epinephrine) & Noradrenaline	Emergency sympathetic "Fight-or-Flight" response: Tachycardia, glycogenolysis, pupillary dilation	Emergency catecholamines.
Endocrine Pancreas (Islets of Langerhans)	$α$ -cells: Glucagon; $β$ -cells: Insulin; $δ$ -cells: Somatostatin	Glucagon raises blood glucose (Hyperglycemic); Insulin lowers blood glucose (Hypoglycemic) via GLUT-4	Diabetes Mellitus (Insulin deficiency/resistance $⟹$ persistent hyperglycemia, glucosuria, ketonuria).

2. ⚡ Mechanism of Hormone Action (Biochemical Pathways)

                       HORMONE RECEPTOR CLASSIFICATION
                                      │
         ┌────────────────────────────┴────────────────────────────┐
         ▼                                                         ▼
   [PEPTIDE / PROTEIN HORMONES]                              [STEROID & THYROID HORMONES]
   (Hydrophilic: Insulin, Glucagon, TSH, FSH, LH, Epinephrine) (Lipophilic: Estrogen, Progesterone, Cortisol, T3/T4)
   ├── Cannot cross lipid bilayer                            ├── Diffuse freely across cell membrane
   ├── Binds to Extracellular Cell-Surface Receptors         ├── Binds to Intracellular / Nuclear Receptors
   ├── Generates Second Messengers:                          ├── Hormone-Receptor Complex binds directly to DNA (HRE)
   │   └── cAMP, IP3, DAG, Ca2+                              └── Regulates Gene Transcription & Protein Synthesis
   └── Fast, transient enzyme cascade modulation             └── Slower, sustained developmental/genomic response