Functions of Thyroid Hormones

June 25, 2022

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.

Image source: thoughtco

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.

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.

Image source: scienceabc

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.