HORMONES – THE BASICS ON THEIR TYPES AND FUNCTION

Hormones are chemical substances secreted by ductless glands (endocrine glands) that are released directly into the circulation in small amounts in response to a specific stimulus and causes response on the target cells or organs. Hormones interact with their target cells through receptors which are large protein molecules with specific binding sites for specific hormones.

Hormones are secreted in extremely low concentration into the circulation, while metabolic end products are secreted in large amounts, thus are called parahormones.

Glands and organs that secrete hormones constitute the endocrine system ( endo – within, krinein – to secrete ). Hormones are called endocrine because they are secreted by ductless glands directly into the bloodstream.

LIST OF HORMONES AND GLANDS THAT SECRETE THEM

GLANDS	HORMONES
ANTERIOR LOBE OF PITUITARY GLAND	Thyroid stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Growth hormone (GH) Follicle stimulating hormone (FSH) Luteinizing hormone (LH) Prolactin
POSTERIOR LOBE OF PITUITARY	Vasopressin (antidiuretic hormone/ADH) Oxytocin
THYROID GLAND	Tri-iodo-thyronine ( T3 ) Thyroxine ( T4 ) Calcitonin
PARATHYROID GLAND	Parathyroid hormone
PANCREAS	Insulin Glucagon Somatostatin
ADRENAL MEDULLA	Epinephrine and norepinephrine
ADRENAL CORTEX	Mineralocorticoids (Aldosterone, deoxycorticosterone) Glucocorticoids (Cortisol or hydrocortisone, corticosterone) Sex steroids (Androgen, oestrogen and progesterone) *Androgens and oestrogen are also secreted by gonads.
PINEAL GLAND	Melatonin

CLASSES OF HORMONES

Hormones are divided into three major classes:

• Steroids
• Proteins and polypeptides
• Amino acid derivatives

STEROIDS

EXAMPLES –

• Mineralocorticoids
• Glucocorticoids
• Sex hormones (androgen, oestrogen, progesterone)
• Vitamin D₃ (1,25-dihydroxycholecalciferol)

Steroids are hydrophobic, lipid soluble substances. Their half life varies from 60 minutes to 100 minutes. They are stored only for few minutes, so, they must be synthesized continuously for constant action.

PROTEINS & POLYPEPTIDE

EXAMPLES

• Anterior and posterior pituitary hormones
• Hypothalamic hormones
• Parathyroid hormone
• Insulin
• Glucagon
• Calcitonin
• Gastrin
• Secretin
• Angiotensin

These are water soluble. Their half life varies from 5 minutes to 60 minutes.

AMINO ACID DERIVATIVES

EXAMPLES

• Epinephrine
• Nor-epinephrine
• Tri-iodothyronine ( T₃ )
• Thyroxine ( T₄ )

Thyroid hormones have half life of 7 – 9 days. Epinephrine and nor-epinephrine have half life of 1 – 3 minutes.

FEW ORGANS WITH ENDOCRINE FUNCTION

ORGAN	HORMONE
HEART	Atrial natriuretic peptide (ANP)
GIT	Gastrin, secretin, cholecystokinin, motilin, ghrelin, somatostatin, vasointestinal peptide
KIDNEY	1, 25-Dihydroxycholecalciferol
SKIN	Calciferol ( vitamin D3 )
LIVER	Insulin like growth factors
PLATELETS	Platelet – derived growth factor ( PDGF)
LYMPHOCYTES	Interleukins

FUNCTIONS OF HORMONES

HORMONE	ACTION
Growth hormone (GH)	Stimulates growth of bone, cartilage and connective tissue.
Prolactin	Plays an important role in the development of mammary gland and synthesis of milk.
Antidiuretic hormone (ADH) / Vasopressin	Stimulates water resorption by kidneys.
Oxytocin	Causes milk ejection from lactating breasts, initiates labor.
Thyroid stimulating hormone (TSH)	Stimulates release of T3 & T4 .
Adrenocorticotropic hormone (ACTH)	Stimulates release of glucocorticoids and androgens from adrenal cortex,
Follicle stimulating hormone (FSH)	Promotes maturation of follicles and estrogen production, stimulates sperm production.
Luteinizing hormone (LH)	Triggers ovulation and production of estrogen and progesterone by ovary, promotes sperm production.
Thyroid hormones	Increases metabolism, regulates tissue growth, increases protein synthesis, decreases serum cholesterol.
Parathyroid hormone	Increases blood calcium levels through action on bone, kidneys and intestine.
Calcitonin	Regulation of blood calcium in levels in children through uptake by bone.
Glucocorticoids	Protein breakdown, increases blood glucose levels, increases gastric acid and pepsin secretion, immune suppression
Mineralocorticoids	Sodium and water retention, excretion of potassium, maintenance of blood volume and blood pressure.
Androgens	Development of secondary sexual characteristics in males, sperm production
Estrogens	Development of secondary sexual characteristics in females, regulation of menstrual cycle
Progesterone	Regulation of menstrual cycle
Epinephrine & norepinephrine	Increase heart rate, increase blood sugar, increase basal metabolic rate, vasoconstriction
Insulin	Decreases blood glucose by increasing its uptake by tissues.
Glucagon	Increases blood glucose.
Melatonin	Regulates circadian rhythm.
Somatostatin	Inhibits release of insulin and glucagon, inhibits release of growth hormone.