Actions of thyroid hormones

Increase basal metabolic rate

• Calorigenic action
• action that increases BMR
• Stimulates O2 consumption of almost all metabolically active tissues
• except adult brain, testes, uterus, lymph nodes, spleen & anterior pituitary
• T4 suppresses O2 consumption of the anterior pituitary
• because it inhibits TSH secretion
• Increase activity of Na/K ATPase in many tissues
• Increase Mitochondrial protein synthesis
• Hyperthyroidism
• BMR 60-100% above normal
• Hypothyroidism
• BMR 20-40% below normal

Increase protein synthesis

• Increase RNA translation
• Increase transcription
• Increase mitochondrion activity
• Increase activity of enzyme

Carbohydrate metabolism

• Increase absorption of glucose from GIT (similar to insulin)
• Increase glucose uptake by the cells
• accelerate glucose transport thru the cell membrane
• Increase breakdown of glycogen to glucose
• Accelerates gluconeogenesis

Lipid metabolism

• Decreases lipid storage
• mobilising it from adipose tissues
• mobilised lipid is then converted to free FA & transported by blood
• Free FA in blood increases
• Decreases cholestreol, phospholipids & TG in plasma
• Hyposecretion: increase cholesterol level – atherosclerosis
• Increase deposition of lipids in liver –> fatty liver
• Decrease plasma cholesterol by increasing excretion from liver cells into bile

Increase heat production in the body

• accelerate cellular metabolic processes
• thyroid hormone induced thermogenesis

Growth

• accelerates growth of body
• especially children
• Growith & development of brain during detal life & first few years of postnatal life

Body weight

• Decrease body weight & fat storage
• Hypothyroidism: fat deposition

Blood

• increase RBC production
• Accelerates erythropoiesis
• icrease dissociation of O2 from hemoglobin
• by increasing 2,3-biphosphoglycerate (BPG)
• For increased activity of tissues

CVS

• Increase no. & affinity of B-adrenergic receptors in the heart
• increase sensitivity to the inotropic (contractility) & chronotropic (rate) effects of cathecholamines

Respiration

• Increase rate & force of respiration
• Increase metabolic rate, increased demand for O2 & formation of CO2
• stimulates respiratory centers to increase rate & force of respiration

GIT

• Increase appetite & food intake
• Increase secretions & movements of GIT
• Hypersecretion
• Diarrhea
• Hyposecretion
• Constipation

CNS

• Development & maintenance of normal function of CNS

Skeletal muscle

• Essential for normal activity of skeletal muscle
• Slight increase in hormone: muscle work more vigorously
• Hypersecretion: muscle weakness
• catabolism of proteins: thyrotoxic myopathy
• Hypersecretion: fine muscular tremor
• due to thyroxine-induced excess neuronal activity
• Hyposecretion: sluggish muscle

Sexual function

• Essential for normal sexual function
• In man
• hypersecretion: impotence
• hyposecretion: loss of libido
• In woman
• hypersecretion: oligomenorrhea, amernorrhea
• hyposecretion: menorrrhagia, polymenorrhea
• some oligomenorrhea, amenorrhea