Exam #4: Thyroid Gland Flashcards Preview

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Flashcards in Exam #4: Thyroid Gland Deck (36):
1

Describe the anatomy of the thyroid gland. What innervates the thyroid gland. What arteries supply the thyroid gland?

The thyroid gland is located just anterior to the thyroid cartilage and is composed to two lobes connected by an isthmus.
- Vasomotor innervation from the SNS
- Superior and inferior thyroid arteries

2

Where and how thyroid hormones are made and secreted?

Thyroid hormone is synthesized within the colloid of thyroid follicles, the functional unit of the thyroid gland. These are the steps involved in the formation of thyroid hormones:

1) Continual synthesis and secretion of thyroglobulin into the follicular lumen
2) Iodide trapping, or concentrating iodide in the follicular epithelial cell, via 2Na+-I- symporter (NIS)
3) Movement of iodide into the follicular lumen via another transporter, pendrin
4) Oxidation of iodide to iodine (I- -->I2)
5) Organificiation, or binding iodine to thyroglobulin, catalyzed by thyroid peroxidase
6) Coupling reactions between MIT & DIT
7) Endocytosis of thryoglobulin via megalin
8) Hydrolysis of T3 & T4 from thyroglobulin by lysosomal enzymes & secretion into blood

3

Describe the functions of the thyroid hormones.

- Generally, thyroid hormone is important for energy homeostasis & regulating energy expenditure; thyroid hormone STIMULATES cell metabolism & activity
- Thyroid hormone is also very important for development, and thyroid deficiencies produce a variety of defects, including severe mental retardation

4

What are the target cells of the thyroid hormones?

Thyroid hormone receptors are found in virtually all tissues

5

Where are thyroid hormone receptors located (on cell surface or intracellular)?

The thyroid hormone receptor is located intracellularly; it is bound to or near the Thyroid Response Element (TRE) in DNA
- Thyroid receptor is associated with the Retinoid X Receptor
- In the absence of thyroid hormone, the TRE is repressed
- Thyroid hormone binding causes gene transcription

6

Describe the effects of too much or too little thyroid hormone.

Thyroid hormone is needed for normal body growth during fetal life, infancy, and childhood
- Too little will result in stunted growth and mental retardation
-

*Because of these long-term implications, thyroid levels should be measured at birth

7

What is cretinism?

This is a form of severe mental retardation that is caused by extreme hypothyroidism in early life

8

Explain why clinicians measure TSH.

TSH is used as an indirect measure of thyroid hormone levels
- It is assumed that if thyroid hormone levels are LOW, TSH levels are HIGH
- The converse is also assumed, if thyroid hormone levels are HIGH, then TSH is LOW

9

Describe the negative feedback circuits that control TSH release.

- Thyroid hormone secretion is under the control of the hypothalamus & pituitary gland
- T3 & T4 inhibit TRH release from the hypothalamus & TSH from the anterior pituitary gland

10

Describe the cause and effects of Graves’ disease.

Grave's disease is the most common cause of hyperthyroidism and is an autoimmune disorder where autoantibodies stimulate TSH receptors on the thyroid gland. Symptoms include:
- Weight loss accompanied by increased food intake
- Excessive heat production & sweating
- Hair loss
- Exophthalmos
- Goiter
- Tachycardia
- Tremor
- Nervousness
- Weakness

11

Describe the cause and effects of Hashimoto's Disease.

Hashimoto's thyroiditis is a common cause of hypothyroidsm, which is the result of autoantibodies that are directed against thyrolobulin or thyroid peroxidase i.e. destroy the thyroid gland or block thyroid hormone synthesis. Symptoms include:
- Weight gain without increased food intake
- Decreased heat production & cold intolerance
- Bradycardia
- Slowed mental activity
- Periorbital puffiness
- Myxedema leading to thickened/ coarse features

12

Describe how thyroid hormones are transported in the blood.

Thyroid hormone is transported in the blood mostly bound to plasma proteins
- Major binding protein is thyroxine-binding globulin (TBG)

13

What is the major thyroid hormone binding protein in the blood?

Thyroxine-binding-globulin (TBG) is the major binding protein in the blood

14

Is there another specific thyroid binding hormone? If there is what is its name?

Transthyretin (TTR) is another "semi-specific" thyroid-binding protein
- Can bind Thyroxine & Retinol

15

What other non-thyroid specific proteins bind and transport thyroid hormones?

Albumin
Lipoproteins

16

What is iodide trapping?

Concentrating iodide in the follicular epithelial cell

17

What is organification?

Attaching iodine to tyrosine residues of thryoglobulin via thyroid peroxidase

18

What is the function of Thyroxine-binding prealbumin?

This is another name for Transthyretin (TTR) that can semi-specifically bind thyroid hormone in the blood (as well as Retinol)

19

What is the function of Transthyretin?

Semi-specific binding thyroid hormone in the blood (as well as Retinol); this binding creates a large reservoir of preformed hormones that are protected from inactivation and excretion

20

What is the function of Thyroglobulin?

Thryglobulin binds T3 & T4 in the colloid of the follicular lumen to store thryroid hormones

21

What is the function of Thyrotropin?

Thyrotropin is another name for Thyroid Stimulating Hormone (TSH), which is released from thrytrophes of the anterior pituitary in response to TRH

TSH binding to the thyroid gland generally initiates the breakdown of thyroglobulin & causes the release of pre-formed thyroid hormone, but it can also
1) Stimulate NIS (2Na+/I- symporter)
2) Increase iodination of tyrosine
3) Increase size & number of thyroid cells

22

What is the function of Thyrotropin-releasing hormone?

Thyrotropin-Releasing Hormone, or TRH is released from the hypothalamus and stimulates secretion of TSH from the anterior pituitary

23

What is the function of Thyroid stimulating hormone?

TSH or Thyrotropin (another name for Thyroid Stimulating Hormone (TSH), is released from thrytrophes of the anterior pituitary in response to TRH from the hypothalamus

TSH binding to the thyroid gland generally initiates the breakdown of thyroglobulin & causes the release of pre-formed thyroid hormone, but it can also
1) Stimulate NIS (2Na+/I- symporter)
2) Increase iodination of tyrosine
3) Increase size & number of thyroid cells

24

Briefly describe a goiter.

Goiter is an abnormal enlargement of your thyroid gland.
- This is a normal physiological response to TSH without enough thyroid hormone production

25

What causes goiter?

- Lack of iodide in the diet is the most common cause of Goiter
- Lack of thyroid hormone

26

State if T4, TSH and TRH increase or decrease in patients with primary hypothyroidism.

In primary hypothyroidism there are low levels of thyroid hormones i.e. T3 & T4
- TSH & TRH are HIGH

In primary hyperthyroidism, there are high levels of thyroid hormones
- TSH & TRH are LOW

27

State if T4, TSH and TRH increase or decrease in patients with pituitary hypothyroidism i.e. secondary hypothyroidism.

In secondary or pituitary hypothyroidism there are low levels of TSH
- T3 & T4 are consequently low as well
- TRH is HIGH

In secondary hyperthyroidism, TSH is HIGH
- T3 & T4 are also high
- TRH is LOW

28

State if T4, TSH and TRH increase or decrease in patients with Hypothalamic hypothyroidism.

Here TRH is low; consequently so is everything else

29

State if T4, TSH and TRH increase or decrease in patients with Grave's Disease.

- Decreased TSH
- Increased T3 & T4
- Increased antibodies to TSH receptor

30

What is the active form of Thyroid hormone?

- T3 is the active hormone
- T4 makes up the majority of the thyorid hormone circulating in the blood; this is a prohormone

31

What is reverse T3?

Inactive from of T3 that occurs when the inner ring is deiodinated instead of the outer ring

32

What is the function of thyroid peroxidase?

Generally, thyroid peroxidase adds iodine to thyroglobulin
- Catalyzes organification & coupling reactions

33

Where does thyroglobulin come from?

Continually synthesized by the RER & Golgi in the follicular epithelial cells

34

What can cause congenital hypothyroidism?

- Maternal iodine deficiency
- Fetal thyroid dysgenesis
- Inborn errors of thyroid hormone synthesis
- Maternal antithyroid antibodies
- Fetal hypopituitary hypothyroidism

35

What are the effects of thyroid hormone on the cardiovascular system?

Increased cardiac rate & output

36

What is one of the important metabolic outcomes of hypothyroidism?

Thyroid hormone stimulates carbohydrate and fat metabolism; a deficiency can cause a rise of cholesterol in the blood

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