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The Endocrine System > Thyroid Physiology > Flashcards

Thyroid Physiology Flashcards

1
Q

Which three hormones does the thyroid produce?

A
  1. Thyroxine (T4)
  2. Triiodothyronine (T3)
  3. Calcitonin
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2
Q

The parathyroid glands are responsible for regulation of what in the body?

A

Calcium

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3
Q

How much does the thyroid gland weigh?

A

25-30g

(adults)

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4
Q

How can the thyroid gland be imaged?

A
  1. USS
  2. Iodine uptake (it is highly vascular)
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5
Q

Describe the cells involved in the structure of the thyroid gland

A

Follicular cells surround colloid

Parafollicular C cells are dispersed rarely

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6
Q

What is a colloid?

A

Mixture of substances

Contains thyroglobulin

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7
Q

How are T3 and T4 synthesised?

A

Iodine is taken up by follicle cells and attaches to tyrosine residues forming monoiodotyrosine (MIT) and di-iodotyrosine (DIT) units.

Coupling of units occurs with MIT + DIT forming T3 and DIT + DIT forming T4

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8
Q

Where are T3 and T4 stored until they are required?

A

Colloid thyroglobulin

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9
Q

Which two drugs are used most to treat hyperthyroidism?

A
  1. Carbimazole
  2. Propylthiouracil (PTU)
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10
Q

How do carbimazole and PTU function?

A

Inhibits binding of iodine to tyrosine residues preventing formation of T3 and T4

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11
Q

Which thyroid hormone comprises around 90% of all secreted thyroid hormones?

A

T4

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12
Q

Which thyroid hormone is more potent?

A

T3

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13
Q

Which enzyme is responsible for the conversion of T4 to T3?

A

Deiodinase enzyme

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14
Q

Where does the conversion of T4 to T3 largely occur?

A

Liver and kidneys

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15
Q

Why do do pharmacotherapies for hyperthyroidism not work instantly?

A

They inhibit formation of T3/4

There is a large reservoir of these hormones already built up in the colloid

To have an effect these hormone stores must first be used up

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16
Q

What induces T3/4 release from follicle cells?

A

TSH binding

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17
Q

Which plasma proteins can T3/T4 bind to?

A
  1. Thyroxine binding globulin (70%)
  2. Transthyretin (20%)
  3. Albumin (5%)
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18
Q

Since T3 is the biologically active form, where does it act upon?

A

Nuclear receptors

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19
Q

What effect does T3 have when bound to nuclear receptors?

A

Alters transcription of certain genes

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20
Q

Metabolic state correlates most directly with what?

A

Free T3/T4

(moreso that total concentration)

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21
Q

Why do thyroid hormones increase basal metabolic rate? (3)

A
  1. Increase no. and size of mitochondria
  2. Increase O2 use and rates of ATP hydrolysis
  3. Increase synthesis of respiratory chain enzymes
22
Q

In which two ways is carbohydrate metabolism increased by thyroid hormones?

A
  1. Blood glucose levels increase as glycogenolysis and gluconeogenesis are stimulated
  2. Insulin-dependent glucose uptake into cells is increased
23
Q

How is protein metabolism impacted by thyroid hormone release?

A

There is increased protein synthesis

24
Q

How do thyroid hormones increase responsiveness to adrenaline and sympathetic neurotransmitters?

A

Increasing no. of receptors

25
Q

In hyperthyroidism, there is cardiovascular responsiveness to rasied thyroid hormones. How can this be treated pharmacologically?

A

Propranolol

26
Q

Why is propranolol the preferred beta blocker for cardiovascular responsiveness to thyroid hormones?

A

It reduces the symptoms of cardiovascular responsiveness

It has inhibitory action upon deiodinase enzymes blocking conversion of T4-T3 which provides a minimal but added therapeutic benefit

27
Q

The thyroid stimulating hormone receptor is which type of receptor?

A

G protein coupled

28
Q

When are thyroid hormones

a) Highest
b) Lowest

A

a) Late at night
b) In the morning

29
Q

How many type of deiodinase enzymes are there?

A

3

30
Q

Where is type I deiodinase enzymes found?

A

Liver and kidney

31
Q

Where are type II deiodinase enzymes found?

A

Heart, skeletal muscle, CNS, fat, thyroid and pituitary

32
Q

Where are type III deiodinase enzymes found?

A
  1. Fetal tissue
  2. Placenta
  3. Brain (not pituitary)
33
Q

Which type of deiodinase enzyme primarily facilitates T4 to T3 conversion in tissues?

A

D2

34
Q

T3 acts on nuclear receptors meaning that disorders affecting the transport of T3 into cells may present with symptoms of what?

A

Hypothyroidism

35
Q

What are the different types of thyroid receptor?

A

Thyroid hormone receptor alpha 1 and 2

Thyroid hormone receptor beta 1 and 2

36
Q

A mutation in THR alpha will lead to what?

A

Defective signalling through the THR-alpha receptor

Resistance to THR-alpha

37
Q

How does THR-alpha resistance present?

A

Phenotype of hypothyroidism

38
Q

How does THR-beta resistance present?

A

Phenotype of hyperthyroidism

39
Q

Why is it that THR-alpha resistance presents with symptoms of hypothyroidism, yet THR-beta resistance presents with symptoms of hyperthyroidism?

A

THR-beta is much more involved in the negative feedback loop.

Resistance leads to much more thyroid hormones (T3/4) being produced.

THR-alpha resistance generally impacts the normal function of the body organs more meaning resistance causes their funtions to “slow”

40
Q

In THR-alpha resistance, how are circulating thyroid hormones affected?

A

Minimally

41
Q

In THR-Beta resistance, how are circulating hormones affected?

A

T3 increased

T4 increased

TSH increased

42
Q

Deficiency of thyroid hormones may be associated with an _____________ thyroid

A

Deficiency of thyroid hormones may be associated with an enlarged thyroid

43
Q

What are some general symptoms of a deficiency of thyroid hormones?

A
  1. Reduced basal metabolic rate
  2. Slow pulse rate
  3. Fatigue and lethargy
  4. Cold intolerance
  5. Tendency to put on weight
44
Q

What is the term given to a deficiency of thyroid hormones in adults?

A

Myxoedema

45
Q

What is the term given to a deficiency in thyroid hormones in babies?

A

Cretinism

46
Q

What is cretinism and how does it present?

A

A deficiency of thyroid hormones at birth

Causes dwarfism and limited mental functioning

47
Q

What is the pathogenesis of Grave’s disease?

A

Thyroid stimulating immunoglobulin acts like TSH, yet is not regulated by T3/T4

48
Q

What is the term given to the bulging eyes seen in Grave’s disease and why does this happen?

A

Exophthalmos

Eyes bulge due to water retaining carbohydrate build up behind the eyes

49
Q

What are some of the general symptoms of hyperthyroidism?

A
  1. Increased basal metabolic rate
  2. Very fast pulse rate
  3. Increased nervousness and emotional state
  4. Isomnia
  5. Sweating/heat intolerance
  6. Tendency to lose weight
50
Q

A goitre can ony be seen in hyperthyroidism

True or false?

A

False

A goitre can be seen in both hyper and hypothyroidism

The Endocrine System (35 decks)

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