Functions of Thyroid Harmones

 FUNCTIONS OF THYROID HORMONES

Thyroid hormones have two major effects on the body:

I. To increase basal metabolic rate

II. To stimulate growth in children.

The actions of thyroid hormones are:

ACTION ON BASAL METABOLIC RATE (BMR)

Thyroxine increases the metabolic activities in most of the body tissues, except brain, retina, spleen, testes and lungs. It increases BMR by increasing the oxygen consumption of the tissues. The action that increases the BMR is called calorigenic action. In hyperthyroidism, BMR increases by about 60%

to 100% above the normal level and in hypothyroidism it falls by 20% to 40% below the normal level.

ACTION ON PROTEIN METABOLISM

Thyroid hormone increases the synthesis of proteins in the cells. The protein synthesis is accelerated by the following ways:

i. By Increasing the Translation of RNA

Thyroid hormone increases the translation of RNA in the cells. Because of this, the ribosomes are activated and more proteins are synthesized.

ii. By Increasing the Transcription of DNA to RNA

Thyroid hormone also stimulates the transcription of DNA to RNA. This in turn accelerates the synthesis of proteins in the cells (see above).

iii. By Increasing the Activity of Mitochondria

In addition to acting at nucleus, thyroid hormone acts at mitochondrial level also. It increases the number

and the activity of mitochondria in most of the cells of the body. Thyroid hormone accelerates the synthesis of RNA and other substances from mitochondria, by activating series of enzymes. In turn, the mitochondria increase the production of ATP, which is utilized for the energy required for cellular activities.

iv. By Increasing the Activity of Cellular Enzymes

Thyroid hormones also increase the activity of at least 100 or more intracellular enzymes such as alphaglycerophosphate dehydrogenase and oxidative enzymes. These enzymes accelerate the metabolism of proteins and the carbohydrates. Though thyroxine increases synthesis of protein, it

also causes catabolism of proteins.

ACTION ON CARBOHYDRATE METABOLISM

Thyroxine stimulates almost all processes involved in the metabolism of carbohydrate.

Thyroxine:

i. Increases the absorption of glucose from GI tract

ii. Enhances the glucose uptake by the cells, by accelerating the transport of glucose through the cell membrane

iii. Increases the breakdown of glycogen into glucose

iv. Accelerates gluconeogenesis.

ACTION ON FAT METABOLISM

Thyroxine decreases the fat storage by mobilizing it from adipose tissues and fat depots. The mobilized

fat is converted into free fatty acid and transported by blood. Thus, thyroxine increases the free fatty acid level in blood.

ACTION ON PLASMA AND LIVER FATS

Even though there is an increase in the blood level of free fatty acids, thyroxine specifically decreases the

cholesterol, phospholipids and triglyceride levels in plasma. So, in hyposecretion of thyroxine, the cholesterol level in plasma increases, resulting in atherosclerosis. Thyroxine also increases deposition of fats in the liver, leading to fatty liver. Thyroxine decreases plasma cholesterol level by increasing its excretion from liver cells into bile. Cholesterol enters the intestine through bile and then it is excreted through the feces.

ACTION ON VITAMIN METABOLISM

Thyroxine increases the formation of many enzymes. Since vitamins form essential parts of the enzymes, it is believed that the vitamins may be utilized during the formation of the enzymes. Hence, vitamin deficiency is possible during hypersecretion of thyroxine.

ACTION ON BODY TEMPERATURE

Thyroid hormone increases the heat production in the body, by accelerating various cellular metabolic processes and increasing BMR. It is called thyroid hormoneinduced thermogenesis. During hypersecretion of thyroxine, the body temperature increases greatly, resulting in excess sweating.

ACTION ON GROWTH

Thyroid hormones have general and specific effects on growth. Increase in thyroxine secretion accelerates the growth of the body, especially in growing children. Lack of thyroxine arrests the growth. At the same time, thyroxine causes early closure of epiphysis. So, the height of the individual may be slightly less in hypothyroidism. Thyroxine is more important to promote growth and development of brain during fetal life and first few years of postnatal life. Deficiency of thyroid hormones during this period leads to mental retardation.

ACTION ON BODY WEIGHT

Thyroxine is essential for maintaining the body weight. Increase in thyroxine secretion decreases the body weight and fat storage. Decrease in thyroxine secretion increases the body weight because of fat deposition.

ACTION ON BLOOD

Thyroxine accelerates erythropoietic activity and increases blood volume. It is one of the important general factors necessary for erythropoiesis. Polycythemia is common in hyperthyroidism.

ACTION ON CARDIOVASCULAR SYSTEM

Thyroxine increases the overall activity of cardiovascular system.

i. On Heart Rate

Thyroxine acts directly on heart and increases the heart rate. It is an important clinical investigation for

diagnosis of hypothyroidism and hyperthyroidism.

ii. On the Force of Contraction of the Heart

Due to its effect on enzymatic activity, thyroxine generally increases the force of contraction of the

heart. But in hyperthyroidism or in thyrotoxicosis, the heart may become weak due to excess activity and

protein catabolism. So, the patient may die of cardiac decompensation.

Cardiac decompensation refers to failure of the heart to maintain adequate circulation associated with

dyspnea, venous engorgement (veins overfilled with blood) and edema.