Three general classes of hormone exist : ?1) Proteins and polypentideMost of hormones in the body are polypeptides and proteins. In general, polypeptides with 100 or more aminoacids are called proteins, and those with fewer than 100 amino acids are referred to as peptides.Proteins and peptide hormones are synthesized on the rough ER. They are usually synthesized first as larger proteins (precursors) that are not biologically active (preprohormone) and cleaved in the ER to form smaller prohormones. These are then transferred to the Golgi apparatus for packing into secretory vesicles. In this process, enzymes in the vesicles cleave the prohormones to produce smaller, biologically active hormones and inactive fragments. Vesicles are stored into cytoplasm and secretion of the hormones occur by exocytosis.The peptide/protein hormones are :?Hypothalamus :- Thyrotropin releasing hormone (TRH), coicotropin-releasing hormone (CRH), Growth-hormone releasing hormone (GHRh), Growth hormone inhibitory hormone (GHIn), i.e., somatostatin, Gonadotropin-releasing hormone (GnRH).Anterior pituitary :- Growth hormone, thyroid stimulating hormone (THS) or thyrotropin, adrenocoicotropin hormone (ACTH), prolactin, follicle stimulating hormone (FSH), leutinizing hormone (LH).Posterior pituitary : - Antidiuretic hormone (ADH)/vasopressin, oxytocin.Thyroid gland : - CalcitoninPancreas : - Insulin, glucagon.Parathyroid : - ParathormonePlacenta : - Human chorionic gonadotropin (HCG), Human somatomammotropin.Kidney : - Renin, erythropoietinHea : - Atrial natriuretic peptide (ANP)Stomach : - GastrinSmall intestine : - Secretin, Cholecystokinin (CCK)Adipocytes : - LeptinAngiotensin IISome of these peptide hormones are glycoproteins, i.e., they have sugar associated with proteins. Glycoprotein hormones are FSH, LH, TSH, human chorionic gonadotropin, erythropoietin.2) Steroid hormonesThese hormones are synthesized from cholesterol. Once they are synthesized, they are simply diffuse across the cell membrane (because they are lipid soluble) and enter the interstitial fluid and then the blood. Thus, they are not stored.Steroid hormones are : ?Adrenal coex : - Coisol (Glucocoicoids), Aldosterone (mineralocoicoids).Gonads : - Testosterone, estrogen, progesterone.Kidney : - 1, 25 - Dihydroxycholecalciferol3) Amine hormonesThese hormones are derivatives of amino acid tyrosine. These hormones are :Thyroid gland : - Thyroxine (T4) and Triiodothyronine (T3).Adrenal medulla : - Norepinephrine (Noradrenaline), Epinephrine (adrenaline)Hypothalamus : - Dopamine or Prolactin inhibiting factor (PIF)