Question

why is the thyroid glad most important in the endocrine system ? Explain why and defend your answer. Your response should be a minimum of 100 words, with in-text citations and references.

Answer 1

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:
1. 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.
2. 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
ii. By Increasing the Transcription of DNA to RNA.   
iii. By Increasing the Activity of Mitochondria.   
iv. By Increasing the Activity of Cellular Enzymes

3. 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. Enhancestheglucoseuptakebythecells,by
accelerating the transport of glucose through
the cell membrane
iii. Increases the breakdown of glycogen into
glucose
iv. Accelerates gluconeogenesis.
4. 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.
5. ACTION ON PLASMA AND LIVER FATS
and increasing BMR. It is called thyroid hormone- induced thermogenesis. During hypersecretion of thyroxine, the body temperature increases greatly, resulting in excess sweating.
8. 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.
9. 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.
10. 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.
11. 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.
6. 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.
7. ACTION ON BODY TEMPERATURE
Thyroid hormone increases the heat production in the body, by accelerating various cellular metabolic processes

12. ACTION ON RESPIRATION
Thyroxine increases the rate and force of respiration indirectly. The increased metabolic rate (caused by thyroxine) increases the demand for oxygen and formation of excess carbon dioxide. These two factors stimulate the respiratory centers to increase the rate and force of respiration .
13. ACTION ON GASTROINTESTINAL TRACT
Generally, thyroxine increases the appetite and food intake. It also increases the secretions and movements of GI tract. So, hypersecretion of thyroxine causes diarrhea and the lack of thyroxine causes constipation.
14. ACTION ON CENTRAL NERVOUS SYSTEM
Thyroxine is very essential for the development and maintenance of normal functioning of central nervous system (CNS).
i. On Development of Central Nervous System
Thyroxine is very important to promote growth and development of the brain during fetal life and during the first few years of postnatal life. Thyroid deficiency in infants results in abnormal development of synapses, defective myelination and mental retardation.
ii. On the Normal Function of Central Nervous System
Thyroxine is a stimulating factor for the central nervous system, particularly the brain. So, the normal functioning of the brain needs the presence of thyroxine. Thyroxine also increases the blood flow to brain.
Thus, during the hypersecretion of thyroxine, there is excess stimulation of the CNS. So, the person is likely to have extreme nervousness and may develop
psychoneurotic problems such as anxiety complexes, excess worries or paranoid thoughts (the persons think without justification that other people are plotting or conspiring against them or harassing them).
Hyposecretion of thyroxine leads to lethargy and somnolence (excess sleep).
15. ACTION ON SKELETAL MUSCLE
Thyroxine is essential for the normal activity of skeletal muscles. Slight increase in thyroxine level makes the muscles to work with more vigor. But, hypersecretion of thyroxine causes weakness of the muscles due to catabolism of proteins. This condition is called thyrotoxic myopathy. The muscles relax very slowly after the contraction. Hyperthyroidism also causes fine muscular tremor. Tremor occurs at the frequency of 10 to 15 times per second. It is due to the thyroxine-induced excess neuronal activity, which controls the muscle. The lack of thyroxine makes the muscles more sluggish.
16. ACTION ON SLEEP
Normal thyroxine level is necessary to maintain normal sleep pattern. Hypersecretion of thyroxine causes excessive stimulation of the muscles and central nervous system. So, the person feels tired, exhausted and feels like sleeping. But, the person cannot sleep because of the stimulatory effect of thyroxine on neurons. On the other hand, hyposecretion of thyroxine causes somnolence.
17. ACTION ON SEXUAL FUNCTION
Normal thyroxine level is essential for normal sexual function. In men, hypothyroidism leads to complete loss of libido (sexual drive) and hyperthyroidism leads to impotence.
In women, hypothyroidism causes menorrhagia and polymenorrhea .In some women, it causes irregular menstruation and occasionally amenorrhea. Hyperthyroidism in women leads to oligomenorrhea and sometimes amenorrhea .
18. ACTION ON OTHER ENDOCRINE GLANDS Because of its metabolic effects, thyroxine increases
the demand for secretion by other endocrine glands.