Functions of Thyroid Hormones

  • An endocrine gland which is bi-lobed, butterfly shaped or shield shaped.
  • This gland secretes different hormones and these hormones have various functions.
  • They increase basal metabolic rate and stimulate growth in children.
  • The functions of various hormones are:

A) Action on basal metabolic rate (BMR)

  • Increases the metabolic activities in most of the body tissues.
  • Brain, spleen, retina, testes and lungs activities are not affected.
  • Oxygen consumption of the tissues increases as BMR increases.
  • Increase in the BMR is called calorigenic action.
  • BMR elevates by about 60% to 100% above the normal level in hyperthyroidism.
  • It drops by 20% to 40% below the normal in hypothyroidism.

The Thyroid Gland in the Endocrine System

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B) Action on protein metabolism

  • Synthesis of proteins in the cells increases.
  • This increment is accelerated by the following ways:

i) Translation of RNA increases

  • RNA translation increases in the cells.
  • This activates the ribosomes which increases protein synthesis.
  • Thus, more proteins are formed.

ii) Transcription of DNA to RNA increases

  • Transcription of DNA to RNA is stimulated by thyroid hormones.
  • This increases the protein synthesis in the cells.

iii) Activity of mitochondria increases

  • Mitochondria also get acted upon by thyroid hormone in addition to the nucleus.
  • This increases the number and activity of mitochondria in most of the cells.
  • Some other substances and RNA synthesis increases along with the activation of series of enzymes.
  • Production of ATP in turn increases in mitochondria.
  • This fulfills the energy required for cellular activities.

iv) The activity of cellular enzymes increases

  • More than 100 intracellular enzymes activity increases by thyroid hormones.
  • Alpha-Glycerophosphate dehydrogenase and oxidative enzymes are major enzymes activated.
  • Metabolism of proteins and carbohydrates increases in presence of these enzymes.
  • Catabolism of protein also occurs along with increment in the synthesis of protein by thyroxine.

C) Action on carbohydrate metabolism

  • All processes involved in carbohydrate metabolism gets stimulated.
  • The absorption of glucose from GI tract increases.
  • Glucose uptake by the cells enhanced as glucose transportation accelerates through the cell membrane.
  • Breakdown of glycogen into glucose increases.
  • Gluconeogenesis increases.

D) Action on fat metabolism

  • Fat storage decreases as it is mobilized from adipose tissues and fat depots.
  • The fat converts to free fatty acid and transported by blood.
  • This increases free fatty acid level in blood.

E) Action on plasma and liver fats

  • Though free fatty acid level increases in blood, cholesterol, phospholipids and triglyceride level decreases.
  • Atherosclerosis results as cholesterol level increases in plasma in case of hypo-secretion.
  • Deposition of fats in liver increases that result to fatty liver.
  • Cholesterol level in plasma decreases as its excretion from liver cells into bile increases.
  • This cholesterol comes out through the feces as it enters the intestine through the bile.

F) Action on vitamin metabolism

  • Many enzymes formation increases by thyroxine.
  • As vitamins form essential parts of the enzymes, it may be utilized during the formation of enzymes.
  • So, vitamin deficiency may occur during hyper-secretion of thyroxine.

G) Action on body temperature

  • Heat production in the body increases as BMR and various cellular metabolic activities increases.
  • It is called thyroid-hormone-induced thermogenesis.
  • Excess sweating occurs as body temperature increases in hyper-secretion of thyroxine.

H) Action on growth

  • Especially in growing children, thyroxine accelerates the growth as its secretion increases.
  • Lack of thyroxine arrests the growth with early closure of epiphysis.
  • So, height will be slightly less in hypothyroidism.
  • Generally, in fetal life, growth and brain development is promoted till first few years of postnatal life.
  • If deficiency occurs in this period of life, mental retardation occurs.

I) Action on body weight

  • Increasing thyroxine level decreases the body weight and fat storage.
  • Decreasing thyroxine level increases the body weight through fat deposition.
  • Thus, normal thyroxine level is required for maintaining the body weight.

Factors that Affect Thyroid Function (The Institute for Functional... |  Download Scientific Diagram                                                                              Image source: researchgate

J) Action on blood

  • Erythropoietic activity and blood volume increases by action of thyroxine.
  • Thus, it becomes one of the important general factors necessary for erythropoiesis.
  • Polycythemia is common in hyperthyroidism.

K) Action on cardiovascular system

Overall activity of cardiovascular system increases by the activity of thyroxine.

  • Heart rate increases as thyroxine acts directly on heart.
  • This helps to diagnose hypothyroidism and hyperthyroidism.
  • Force of contraction on the heart increases as enzymatic activity is effected by thyroxine.
  • Heart becomes weak due to excess activity and protein catabolism in hyperthyroidism or in thyrotoxicosis.
  • This cause death of the patient which is called cardiac decompensation.
  • Cardiac decompensation means the failure of heart to maintain adequate circulation in certain conditions.
  • The conditions can be dyspnea, venous engorgement and edema.
  • Increase in metabolic activities results to vasodilation.
  • A large quantity of metabolites produced is the reason for vasodilation.
  • Cardiac output finally increases as rate and force of contraction, blood volume and blood flow increases by the influence of thyroxine.
  • This result in increasing blood pressure but the mean pressure is not altered.
  • Only systolic pressure increases whereas diastolic pressure decreases.
  • So, pulse pressure only increases.

L) Action on respiration

  • The rate and force of respiration increases indirectly by activity of thyroxine.
  • As there is increase in metabolic rate, the demand for oxygen also increases and formation of excess carbon dioxide also occurs.
  • This stimulates the respiratory centers which increases the rate and force of respiration.

M) Action on gastrointestinal tract

  • Appetite and food intake increases by thyroxine activity.
  • Various secretions along with the movements of GI tract increases.
  • So, diarrhea may occur in hyperthyroidism and constipation in hypothyroidism.

N) Action on skeletal muscles

  • Normal activity of skeletal muscle is maintained by thyroxine.
  • Muscles work with more vigor in slight increase of this hormone.
  • In hyper-secretion, weakness of muscles occurs due to catabolism of proteins.
  • This condition is called thyrotoxic myopathy.
  • Muscles take time to relax after contraction.
  • Fine muscular tremor also results in hyperthyroidism.
  • Tremor occurs 10-15 times per second which is due to thyroxine induced excess neuronal activity.
  • This neuron generally controls the muscle.
  • Muscles become more sluggish in lack of thyroxine.

O) Action on sleep

  • Normal sleep pattern is maintained in normal thyroxine level.
  • Hyper-secretion stimulates muscles and central nervous system excessively.
  • This result to tiredness, exhaustion and feels like sleeping.
  • Though feels like sleeping develops, stimulatory effect of thyroxine on neurons won’t allow us to sleep.
  • Somnolence results in case of hypo-secretion.

P) Action on sexual function

  • Normal sexual function is maintained by normal level of thyroxine.
  • Complete loss of libido (sexual drive) occurs in male in less secretion and impotence in excess of thyroxine.
  • Menorrhagia and poly-menorrhea is noticed in women in less secretion.
  • In some cases, occasional amenorrhea and irregular menstruation is observed.
  • Excess of thyroxine causes oligo-menorrhea and sometimes amenorrhea.

Thyroid Gland: Function, Location, Hormones, Histology and Anatomy

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Q) Action on other endocrine glands

  • Metabolic effects of these hormones increase the demand for secretion by other endocrine glands.

R) Action on central nervous system

  • Normal maintenance and functioning of CNS is associated with normal level of these hormones.
  • During fetal life and first few years of postnatal life, thyroxine helps to promote growth and development of brain.
  • Abnormal development of synapses results in infants along with defective myelination and mental retardation in its deficiency.
  • Thyroxine also stimulates our brain as it is one of the stimulating factors.
  • Blood flow also increases to brain by this hormone.
  • Excess stimulation of CNS results to nervousness and development of psychoneurotic problems such as anxiety complexes, excess worries or paranoid thoughts.
  • These conditions result if hyper-secretion of thyroxine occurs.
  • Lethargy and somnolence occurs in deficiency of these hormones.





Functions of Thyroid Hormones