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The Endocrine System > Thyroid Physiology > Flashcards

Thyroid Physiology Flashcards

(50 cards)

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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)

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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?

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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?

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A

There is increased protein synthesis

24
Q

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

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A

Increasing no. of receptors

25
In hyperthyroidism, there is cardiovascular responsiveness to rasied thyroid hormones. How can this be treated pharmacologically?
Propranolol
26
Why is propranolol the preferred beta blocker for cardiovascular responsiveness to thyroid hormones?
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
The thyroid stimulating hormone receptor is which type of receptor?
G protein coupled
28
When are thyroid hormones a) Highest b) Lowest
a) Late at night b) In the morning
29
How many type of deiodinase enzymes are there?
3
30
Where is type I deiodinase enzymes found?
Liver and kidney
31
Where are type II deiodinase enzymes found?
Heart, skeletal muscle, CNS, fat, thyroid and pituitary
32
Where are type III deiodinase enzymes found?
1. Fetal tissue 2. Placenta 3. Brain (not pituitary)
33
Which type of deiodinase enzyme primarily facilitates T4 to T3 conversion in tissues?
D2
34
T3 acts on nuclear receptors meaning that disorders affecting the transport of T3 into cells may present with symptoms of what?
Hypothyroidism
35
What are the different types of thyroid receptor?
Thyroid hormone receptor alpha 1 and 2 Thyroid hormone receptor beta 1 and 2
36
A mutation in THR alpha will lead to what?
Defective signalling through the THR-alpha receptor ## Footnote **Resistance to THR-alpha**
37
How does THR-alpha resistance present?
Phenotype of **hypothyroidism**
38
How does THR-beta resistance present?
Phenotype of **hyperthyroidism**
39
Why is it that THR-alpha resistance presents with symptoms of hypothyroidism, yet THR-beta resistance presents with symptoms of hyperthyroidism?
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
In THR-alpha resistance, how are circulating thyroid hormones affected?
Minimally
41
In THR-Beta resistance, how are circulating hormones affected?
T3 increased T4 increased TSH increased
42
Deficiency of thyroid hormones may be associated with an _____________ thyroid
Deficiency of thyroid hormones may be associated with an **enlarged** thyroid
43
What are some general symptoms of a deficiency of thyroid hormones?
1. Reduced basal metabolic rate 2. Slow pulse rate 3. Fatigue and lethargy 4. Cold intolerance 5. Tendency to put on weight
44
What is the term given to a deficiency of thyroid hormones in adults?
Myxoedema
45
What is the term given to a deficiency in thyroid hormones in babies?
Cretinism
46
What is cretinism and how does it present?
A **deficiency** of thyroid hormones at birth Causes **dwarfism** and limited mental functioning
47
What is the pathogenesis of Grave's disease?
Thyroid stimulating immunoglobulin acts like TSH, yet is not regulated by T3/T4
48
What is the term given to the bulging eyes seen in Grave's disease and why does this happen?
Exophthalmos Eyes bulge due to water retaining carbohydrate build up behind the eyes
49
What are some of the general symptoms of hyperthyroidism?
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
A goitre can ony be seen in hyperthyroidism True or false?
False A goitre can be seen in both hyper and hypothyroidism

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