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.
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