Physiology Of The Thyroid Flashcards

1
Q

Describe the hypothalamic-pituitary-thyroid axis

A
  1. Hypothalamus produces TRH
  2. TRH stimulates anterior pituitary to produce TSH
  3. TSH binds to TSH GPCR on surface of thyroid epithelial cells, T3 & T4 released
  4. Negative feedback loop!
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2
Q

Describe the basic structure of the thyroid

A

Follicles - have an inner colloid that is lined by cuboidal epithelial cells & a basement membrane

Parafollicular (C) cells - in between the base membrane, secrete calcitonin

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3
Q

Where is thyroglobulin synthesised

A

Follicular (epithelial) cells

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4
Q

Where is thyroglobulin stored

A

The colloid

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5
Q

What happens to iodide once it is taken up by the epithelial follicular cells

A
  • In epithelial cells - iodide is oxidised to iodine
  • In colloid - iodine attaches to tyrosine residues on thyroglobulin to form DIT & MIT
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6
Q

What makes up T3 vs T4

A

T3 - DIT & MIT
T4 - DIT & DIT

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7
Q

Briefly summarise the formation of T3 & T4 in the colloid upon stimulation by TSH

A

Iodine attaches to tyrosine residues on thyroglobulin
This forms DIT & MIT, which are the building blocks of T3/4

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8
Q

Where does iodine attach on thyroglobulin

A

Tyrosine residues

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9
Q

Is T3 or T4 biologically active

A

T3

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10
Q

Where is T4 activated i.e. converted to T3

A

Kidneys & liver

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11
Q

How is most of thyroid hormones transported in the blood

A

Thyroxine binding globulin (70%)

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12
Q

When considering a patients metabolic state, would it be more helpful to test for free thyroid hormone levels or the total thyroid hormone concentration (including those bound to proteins). Why?

A

Free thyroid hormone levels
- protein bound hormones cant enter cells & have an effect
- throxine bound globulin levels are altered in different conditions

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13
Q

Altered TBG levels can affect the total T4 levels. What would increase TBG?

A

Pregnancy and oral contraceptive pill
Chronic active hepatitis and biliary cirrhosis

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14
Q

Altered TBG levels can affect the total T4 levels. What would decrease TBG?

A

Androgens
Cushing’s
Severe systemic illness
Chronic liver disease

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15
Q

What effect does thyroid have on the liver

A

Increased glycogenolysis and gluconeogenesis
Decreased glycogenesis

I.e. increased glucose use & decreased glucose storage

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16
Q

What effect does thyroid have on the adipose tissue

A
  • Increased lipolysis
  • Decreased lipogenesis

I.e. fat burning

17
Q

What effect does thyroid have on the lungs & heart

A

Increased RR & HR & contraction

=> increased O2 for metabolism

18
Q

What effect does thyroid have on metabolic rate & thermogenesis

A
  • Increased BMR
    (increased mitochondria & aerobic metabolism)
    (increased protein, lipid & carbs metabolism)
  • Increased thermogenesis
19
Q

Summarise the effects of thyroid

A

Increased BMR
- Increased HR & RR => increased O2
- Increased glucose use & decreased storage
- Increased lipid, carbs & protein metabolism (use)

20
Q

What is the relationship between thyroid hormones & growth hormone

A

GHRH production & secretion requires thyroid hormones
GH activity requires thyroid hormones

21
Q

What is the importance of thyroid hormone in the development of foetal & neonatal brain

A

Thyroid hormone required for myelinogenesis & atonal growth

22
Q

What is the relationship between thyroid hormones & adrenaline/noradrenaline

A

Thyroid hormones increase responsiveness of adrenaline and noradrenaline by increasing the number of receptors

23
Q

What enzymes are involved in the degradation of thyroid hormones

A

De-iodinases