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GI Physiology > Lecture 4 - Thyroid gland > Flashcards

Lecture 4 - Thyroid gland Flashcards

1
Q

What are the thyroid hormones?

A

Thyroxine (T4) and triiodothyronine (T3)

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2
Q

What is the function of the thyroid hormones?

A

Regulate metabolic rate and normal growth and development

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3
Q

What is the weight of a normal thyroid gland vs a gioter (enlarged)?

A

Normal is 20 g and goiter is 100 g.

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4
Q

What is the vascularity of the thyroid?

A

Highly vascular with 4-6 ml/min/g of blood going to it so that it can sample blood regularly and secrete hormones quickly.

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5
Q

How many lobes does the thyroid gland have?

A

2 lobes and connected by thin isthmus

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6
Q

The thyroid has closely packed …..

A

acini or follicles

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7
Q

What is the wall of the thyroid made up of?

A

Single layer of cells that if:
columnar - active
cuboidal/low coloumnar - inactive

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8
Q

What is the interior of the thyroid made up of?

A

Clear proteinaceous colloid that is thick and has protein with iodine attached to it.

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9
Q

The acitivity of individual lobules in the thyroid can ……..

A

vary because it is autonomous and they are reandomly controlled and seperately controlled.

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10
Q

What happens if the iodide molecule is not attached in the correct area in the moelcule?

A

Then the molecule becomes inactive for example reverse T3 as oppose to T3.

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11
Q

What are the sources of iodide?

A

Eggs, seafood, salt

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12
Q

How much iodide do we need per week?

A

1mg/week

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13
Q

WHat is iodide trapping?

A

Secondary active transport mechanism where there is an uptake of iodide from the capillary into the follicular cell of the gland. More specifically, the sodium potassium pump which is dependant on ATPase. Essentially, iodide is brought into the cells with sodium.

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14
Q

What is iodide oxidation?

A

It is the process to convert iodide to iodine. IT is catalysed by peroxidase enzymes at the cell colloid itnerface.

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15
Q

What is thyroglobulin ?

A

It is a glycoprotein with a molecular weight of 660,000. It has 2 identical subunits.

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16
Q

How many tyrosine residues are there and how many are iodinated and how many go on to form iodo-thyronine?

A

There are 132 tyrosine residues and only 25-30 are iodinated of which only 6-8 go on to form iodo-thyronine

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17
Q

How many moelcules of T4 and T3 are made per thyroglobulin?

A

3-4 molecules of T4
Only 1 in 5 molecules have T3.
We have an excess of tyrosine residues.

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18
Q

What is organification of I2?

A

I2 is coupled with tyrosine residues of thyroglobulin.

19
Q

If one iodine bonds to the residues then it is called?

A

mono-iodo-tyrosine (MIT)

20
Q

If two iodine bonds to the residues then it is called?

A

di-iodo-tyrosine (DIT)

21
Q

How is t4 formed?

A

coupling of two di-iodo-tyrosine

22
Q

how is t3 formed ?

A

coupling of one mono-iodo-tyrosine and one di-iodo-tyrosine

23
Q

How are the thyroid hormones released in the body?

A

It is released into the body by endocytosis of the colloid. Usually the most recently synthesized is used first.

Essentially, they fuse with lysosomes which the acid proteases breaks down the thyroglobulin. Iodide is conserved by deiodination of MIT and DIT.

24
Q

How much T4 and T3 is released into the body everyday?

A

T4 - 80 micrograms and 0.03 % circulating is free
T3 - 4 micrograms and 0.3 % ciruclating is free

Hence, T3 is more potent and their release is stimulated by TSH.

25
Q

What are the thyroid hormone binding proteins?

A
  1. ) Thyroxin-binding globulin (TBG)
  2. ) Thyroxin Binding Pre-albumin (TBPA)
  3. ) Albumin
26
Q

Where are the thyroid hormones degraded and how long does it take to clear T3 and T4?

A

degraded in the liver and T4 - 6-7 days and T3 - 1 day

27
Q

What is TRH and TSH?

A

TRH - Thyrophin releasing hormone

TSH - Throid stimulating hormone

28
Q

Hypothalamic Pituitary Thyroid Axis?

A
  1. ) Hypothalamus
  2. ) TRH
  3. ) Pituitary Gland
  4. ) TSH
  5. ) Thyroid Gland
  6. ) Thyroid gland releases T4 and T3. T4 can be sent to the liver and converted to T3. Iodine released

negative feedback system - if too much T3 and T4 is circulating around the body than it will inhibit the hypothalamus and pituitary to stop producing signlas for more.

29
Q

What if there is a too much T3 and T4 but low TSH?

A

Problem with thyroid

30
Q

What if there is too much TSH but low T3 and T4?

A

Problem with pituitary

31
Q

What is TRH?

A

It is released by the hypothalamus and acts using Ca2+ as a second messenger. It increases the secretion of thyroid stimulating hormone and also stimulates the release of prolactin. It is rapidly degraded in the plasma.

32
Q

What is TSH?

A

From the anterior pituitary and it is a glycoprotein. It has two peptide chains, one alpha and one beta. The alpha chain is identical to that in LH, FSH, and hCG so the specificity of the molecule comes from the beta chain.

33
Q

What are the actions of TSH?

A
  1. ) increases iodide uptake
  2. ) increases synthesis and secretion of T3 and T4
  3. ) increases vascularity and hypertrophy of thyroid gland.
34
Q

What are the actions of the thyroid hormones?

A

1.) increase basal metabolic rate:
euthyroid (normal) - 30-40Kcal/m2
Hyperthyroid - 60-65Kcal/m2
Hypothyroid - 20-25KCal/m2

2.) Increases oxygen consumption and heat consumption by tissues. Only exceptions are the brain, spleen, testes and ovary. there is a thermogenic effect that is moderated by an increase in blood flow to the skin, increase sweating, and increase ventilation rate.

  1. ) increases muscle protein and fat breakdown
  2. ) increases all aspect of carb metabolism
  3. ) increases GI motility
  4. ) increases heart rate and contractility

many other things

Reason: increase Na+/K+ dependent ATPase

35
Q

What is the action of thyroid hormones in pre natal growth and development and post natal?

A

Pre natal - not essential for growth but it is required for repoductive, skeletal and CNS system.

Post natal - required for normal growth and development. Acts synergistically with GH on bone growth.

36
Q

What happens in primary hypothyroidism?

A

Lack of iodide because:
Decrease in T3 and decreased negative feedback. There is an increase in TSH and TRH and so growth of the thyroid - goiter.

37
Q

Is primary hypothyroidism reversible in infants vs adults or juvenile?

A

Infants - non reversible, causes a dwarfed structure and poor bone structure, muscle weakness. Also mental deficiency, slow pulse, and GI disturbences

Juvenile or adults - reversible, chronic thyroiditis (hashimotos) an autoimmune destruction of the thyroid gland.

Sub acute thyroiditis - thyroid inflammation and initally there is hyperthyroidism and then hypothyroidism. It is reversible but can also lead to permanent hypothyroidism.

38
Q

What is secondary and tertiary hypothyroidism?

A

secondary - pituitary disease where the TSH is not biologically active . Could also be selective TSH deficiency in which there is an autoimmune destruction of TSH secreting cells.

Tertiary - hypothalamic disease may be because of tumor or radiation.

39
Q

Clinical findings of those with hypothyroidism?

A

Dry skin, dry hair, hair loss because there is decreased sebaceous gland activity which produced oil. Also decreased cardiac output - bradycardia, decreased reflexes and mental function and appetite.

Weight gain and always cold and tired.

40
Q

What are some examples of hyperthyroidism (Thyrotoxicosis)?

A

lithium which is used in bipolar treatment inhibits the function of the thyroid

  1. ) excess ingestion of thyroid hormones
  2. ) Graves disease - most common (60%) - autoimmune disease, no feeback with TSH receptors, exophthalmus
  3. ) Toxic multinodular goiter - autonomy growth by a benign adenoma
  4. ) hyperthyroid phase of thyroiditis
41
Q

what is exophthalmus?

A

eyeball is pushed forward in graves disease.

42
Q

What are the symptoms of hyperthyroidism?

A
  1. ) diffuse goitre
  2. ) Exophthalmus
  3. ) Increased BMR, appetite
  4. ) Weight loss and always hot and sweating
  5. ) increased sytolic BP and decreased Diastolic BP
  6. ) Increased nervous alertness and excitability.
  7. ) irregular menses and infertility
  8. ) Increased GI movements
43
Q

Treatments for hyperthyroidism?

A
  1. ) beta blockers - only relieve symptoms (not disease)
  2. ) Thionamides - inhibit thyroid hormone synthesis
  3. ) Radioactive iodine ablation (I131) - if cancerous, radioactive iodine will go to thyroid because that is where 99 % of the iodine in the body is stored.
  4. ) Perchlorate - inhibit iodide trapping
  5. ) 6-n-Propylthiouracil - inhibit coupling
  6. ) natural goitrogens - cabbages, kale
  7. ) lithium - inhibit thyroid hormone release

GI Physiology (16 decks)

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