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Endocrine System > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

1
Q

What does the thyroid gland weigh?

A

20 to 40 g

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2
Q

Secretions of the follicular cells:

A
  1. Thyroglobulin (a colloidal substance)
  2. Tetraiodothyronine or Thyroxine or T4
  3. Triiodothyronine or T3
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3
Q

Hormones secreted by the thyroid gland:

A
  1. T4 (90%)
  2. T3 (9 to 10%)
  3. Calcitonin
  4. Reverse T3
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4
Q

What amino acid do both T4 and T3 contain?

A

They are both iodine-containing derivatives of Tyrosine

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5
Q

Under sufficient supply of Iodine, the T4 to T3 ratio is?

A

7:1

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6
Q

Explain the potency and duration of action of both T4 and T3

A
  1. The potency of T3 is four times more than that of T4.
  2. T4 acts for longer period than T3.
  3. Duration of T4 action is four times more than T3 action. This is because of the difference in the affinity of these hormones to plasma proteins.
  4. T3 has less affinity for plasma proteins and combines loosely with them, so that it is released quickly.
  5. T4 has more affinity and strongly binds with plasma proteins, so that it is released slowly.
  6. Therefore, T3 acts on the target cells immediately and T4 acts slowly.
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7
Q

What is the half life of:
a. T4
b. T3

A

a. 7 days
b. 10 to 24 hours

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8
Q

What is the rate of secretion of the following in a day:
a. T4
b. T3
c. Reverse T3

A

a. 80 to 90 μg/day.
b. 4 to 5 μg/day.
c. 1 to 2 μg/day.

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9
Q

What is the plasma level of:
a. T4
b. T3

A

a. 0.12 μg/dL
b. 8 μg/dL

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10
Q

Where does metabolism/degradation of thyroid hormones take place?

A
  1. Muscles
  2. Liver
  3. Kidney
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11
Q

Where does synthesis of thyroid hormones take place?

A

Thyroglobulin in the follicular cavity

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12
Q

Outline the stages of synthesis of thyroid hormones:

A

Synthesis of thyroid hormones occurs in five stages:
1. Thyroglobulin synthesis
2. Iodide trapping
3. Oxidation of iodide
4. Transport of iodine into follicular cavity
5. Iodination of tyrosine
6. Coupling reactions.

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13
Q

Explain the first stage of synthesis of Thyroid hormones:

A

Endoplasmic reticulum and Golgi apparatus in the follicular cells of thyroid gland synthesize and secrete thyroglobulin continuously.
Thyroglobulin molecule is a large glycoprotein containing 140 molecules of amino acid tyrosine.
After synthesis, thyroglobulin is stored in the follicle.

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14
Q

What is Iodide Trapping?

A

Iodide is actively transported from blood into follicular cell, against an electrochemical gradient, in a process known as iodide trapping.
Iodide is transported into the follicular cell along with sodium by sodium-iodide symport pump or iodide pump.
Normally, iodide is 30 times more concentrated in the thyroid gland than in the blood. However, during hyperactivity of the thyroid gland, the concentration of iodide increases 200 times more.

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15
Q

What enzyme is used during the oxidation of Iodide?

A

Thyroid peroxidase

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16
Q

Why should iodide be oxidised during synthesis of thyroid hormones?

A

Iodide must be oxidized to elementary iodine, because only iodine is capable of combining with tyrosine to form thyroid hormones.
Oxidation of iodide into iodine occurs inside the follicular cells in the presence of thyroid peroxidase. Absence or inactivity of this enzyme stops the synthesis of thyroid hormones.

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17
Q

What is the function of an Iodide-chloride pump?

A

It transports iodine from the follicular cells into the follicular cavity.
The pump is called pendrin

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18
Q

What is iodination and where does it take place?

A

Combination of iodine with tyrosine
It takes place in the thyroglobulin

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19
Q

Define organification

A

Organification of thyroglobulin takes place in the follicular cavity, when thyroglobulin binds to iodine.

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20
Q

Describe iodination of tyrosine

A
  1. Organification of thyroglobulin (binding of thyroglobulin to iodine) takes place first in the follicular cavity.
  2. Then, iodine(I) combines with tyrosine(already present in the thyroglobulin)
  3. This process is accelerated by the enzyme Iodinase
  4. Iodination takes place in several stages; the first stage is when tyrosine is iodised first into mono-iodotyrosine (MIT), then later into di-iodotyrosine (DIT)
21
Q

What enzyme accelerates the process of iodination of tyrosine, and where is it secreted?

A

Iodinase accelerates the process, and it is secreted by the follicular cells

22
Q

What are the iodotyrosine residues?

A
  1. MIT
  2. DIT
23
Q

What are coupling reactions?

A

Occurs when iodotyrosine residues are combined (coupled) to give rise to different thyroid hormones

24
Q

List 5 coupling reactions:

A
  1. Tyrosine + Iodine = Mono-iodotyrosine (MIT)
  2. MIT + I = Di-iodotyrosine (DIT)
  3. DIT + MIT = Tri-iodotyrosine (T3)
  4. MIT + DIT = Reverse T3 (only 1% of thyroid output)
  5. DIT + DIT = Tetra-iodotyrosine (T4)
25
Q

How long can the thyroid gland store its hormones?

A

They are stored in the form of vesicles for up to 4 months
It is the only endocrine gland that can do this
Therefore, if synthesis of thyroid hormones stops, signs and symptoms of deficiency occur after 4 months

26
Q

How many molecules of thyroxine are in a thyroglobulin molecule?

A

5 or 6

There is also an average of 1 T3 for every 10 T4 molecules

27
Q

What mediates the process in which follicular cells send pseudopods to the thyroglobulin-hormone complex?

A

A receptor-like substance called megalin

28
Q

Describe the sequence of events that takes place during the release of thyroid hormones from the thyroid gland.

A

Hormones are first cleaved from thyroglobulin before being released into blood.

  1. Follicular cell sends foot-like extensions called pseudopods, which close around the thyroglobulin- hormone complex. This process is mediated by a receptor-like substance called megalin, which is present in the membrane of follicular cell
  2. Pseudopods convert thyroglobulin-hormone complex into small pinocytic vesicles
  3. Then, lysosomes of the cell fuse with these vesicles
  4. Digestive enzymes such as proteinases present in lysosomes digest (proteolysis) the thyroglobulin and release the hormones
  5. Now, the hormones diffuse through base of the follicular cell and enter the capillaries.

Only T3 and T4 are released to the blood

29
Q

What enzyme deiodinates iodotyrosine residues?

A

Iodotyrosine deiodinase
It results in the release of iodine, which is reutilised in the synthesis of more thyroid hormones by follicular cells

30
Q

List the proteins that transport Thyroid hormones in the blood.

A
  1. Thyroxine-binding Globulin (TBG)
    . Glycoprotein
    . Concentration in blood is 1 to 1.5 mg/dL.
    . Great affinity for thyroxine and about one third of the hormone combines strongly with this protein.
  2. Thyroxine-binding Prealbumin (TBPA)
    . Transports one fourth of the thyroid hormones.
    . Also called transthyretin (TTR).
  3. Albumin
    . Transports about one tenth of the thyroid
    hormones.
31
Q

Functions of Thyroid Hormones

A
  1. To increase the Basal Metabolic Rate
  2. To stimulate growth in children.
32
Q

List the actions of Thyroid Hormones:
a. On BMR

A

Increase the BMR (calorigenic action) by increasing oxygen consumption of the tissues except in brain, retina, spleen, testes and lung

33
Q

List the actions of Thyroid Hormones:
b. On Protein Metabolism

A
  1. By increasing translation of RNA
  2. By increasing transcription of DNA to RNA
  3. By increasing the activity of mitochondria
  4. By increasing the activity of cellular enzymes eg. alphaglycerophosphate dehydrogenase
34
Q

List the actions of Thyroid Hormones:
c. On Carbohydrate Metabolism

A

Thyroxine:
i. Increases the absorption of glucose from GIT
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.

35
Q

List the actions of Thyroid Hormones:
d. On Fat Metabolism

A

It decreases fat storage by mobilising it from adipose tissue and fat depots.
The mobilised fat is converted into free fatty acid and transported in blood.
Thyroxine therefore increases the level of free fatty acids in the blood

36
Q

List the actions of Thyroid Hormones:
e. On Plasma and Liver Fats

A
  1. 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.
  2. Thyroxine also increases deposition of fats in the liver, leading to fatty liver.
  3. 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.
37
Q

List the actions of Thyroid Hormones:
f. On Vitamin Metabolism

A

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.

38
Q

List the actions of Thyroid Hormones:
g. On Body Temperature

A

Thyroid hormone increases the heat production in the body, by accelerating various cellular metabolic processes and increasing BMR (thyroid hormone- induced thermogenesis).
During hypersecretion of thyroxine, the body temperature increases greatly, resulting in excess sweating.

39
Q

List the actions of Thyroid Hormones:
h. On Growth

A
  1. Increase in thyroxine secretion accelerates the growth of the body, especially in growing children.
  2. 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.
  3. 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.
40
Q

List the actions of Thyroid Hormones:
i.On Body Weight

A

Thyroxine is important in maintaining body weight.
1. Increase decreases body weight and fat storage
2. Decrease increases body weight due to fat deposition.

41
Q

List the actions of Thyroid Hormones:
j. On Blood

A

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

42
Q

List the actions of Thyroid Hormones:
k. On Cardiovascular System

A
  1. 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.
  2. 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.
  3. On Blood Vessels
    Thyroxine causes vasodilatation by increasing the metabolic activities. During increased metabolic activities, a large quantity of metabolites is produced. These metabolites cause vasodilatation.
  4. On Arterial Blood Pressure
    Because of increase in rate and force of contraction of the heart, increase in blood volume and blood flow by the influence of thyroxine, cardiac output increases. This in turn, increases the blood pressure. But, generally, the mean pressure is not altered. Systolic pressure increases and the diastolic pressure decreases. So, only the pulse pressure increases
43
Q

List the actions of Thyroid Hormones:
l. On respiration

A

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

44
Q

List the actions of Thyroid Hormones:
m. On GIT

A

Thyroxine increases the appetite and food intake.
2. It also increases the secretions and movements of GIT.
So, hypersecretion of thyroxine causes diarrhea and the lack of thyroxine causes constipation.

45
Q

List the actions of Thyroid Hormones:
n. On Central Nervous Stem

A

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. Its 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 CNS, 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).

46
Q

List the actions of Thyroid Hormones:
o. On Skeletal Muscle

A

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 (thyrotoxic myopathy). The muscles relax very slowly after the contraction. Hyperthyroidism also causes fine muscular tremor, which 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.

47
Q

List the actions of Thyroid Hormones:
p. On sleep

A

Normal thyroxine level is necessary to maintain normal sleep pattern.
Hypersecretion of thyroxine causes excessive stimulation of the muscles and CNS.
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.

48
Q

List the actions of Thyroid Hormones:
r. On other endocrine glands

A

Because of its metabolic effects, thyroxine increases
the demand for secretion by other endocrine glands.

49
Q

List the actions of Thyroid Hormones:
q. On sexual function

A

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.

Endocrine System (3 decks)

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