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Theme 1 - 203 > Thyroid Hormone Physiology > Flashcards

Thyroid Hormone Physiology Flashcards

1
Q

What is the isthmus?

A

Part if the thyroid gland that wraps around the trachea

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2
Q

What is the thyroid gland made up of?

A

Follicles that each consist of a monolayer of epithelial cells that enclose a large core of visors, homogenous colloid.

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3
Q

What is colloid?

A

A reservoir for thyroid hormone

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4
Q

What hormones does the thyroid gland release?

A
  • Triiodothyronine (T3)
  • Tetraiodothyronine (T4)
  • Calcitonin
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5
Q

What is tyrosine and what is it a precursor of?

A

An amino acid and it is a precursor or dopamine

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6
Q

What reactions are involved in the synthesis of T3 and T4?

A

Iodine + Tyrosine —> Monoiodotyrosine (MIT) + Diiodotyrosine (DIT)

MIT + DIT ——> Triiodothyronine (T3)
DIT + DIT ——> Tetraiodothyronine (T4)

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7
Q

What is T4 also known as?

A

Thyroxine

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8
Q

How can T4 get converted into T3 and where does this occur?

A

Iodothyronine deiodinase converts T4 into T3 in tissues

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9
Q

What effect does Fe have on thyroid hormones?

A

Increase

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10
Q

What can happen to MIT and DIT once it is formed?

A

Halogenases can rapidly degrade them to free the iodide, which then gets re-utilised through combination with thyroglobulin.

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11
Q

What is thyroglobulin a precursor of?

A

T3 and T4

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12
Q

What percentage of thyroid hormones released by the thyroid gland are T3 and T4 and which of the two is more biologically active?

A

95% T4
5% T3

T3 is a lot more biologically active

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13
Q

What happens to T4 once it enters target tissues?

A

Gets converted into T3 (80%) or reverse-T3 (20%)

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14
Q

What is the half-life of T3 and T4

A

T3 = 1 day

T4 = 6-8 days

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15
Q

How does the action of the hypothalamus lead to the release of thyroid hormones?

A

1) Hypothalamus releases thyrotropin releasing hormone (TRH)
2) TRH then stimulates the release of thyroid stimulating hormone (TSH) from the anterior pituitary
3) TSH then stimulates the release of T3 and T4 from the thyroid gland

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16
Q

What is goitre?

A

Swelling of the neck due to an enlarged thyroid gland which is due to an iodine deficiency.

It is also known as Derbyshire neck

17
Q

Why are thyroid hormones transported bound to plasma proteins?

A

Because they are insoluble in water

18
Q

What plasma proteins carry thyroid hormones?

A
  • Thyronin-binding globulin (TBG) = 75% of T4
  • Thyroxine-binding prealbumin (TBPA) = 15-20% of T4
  • Albumin = 5-10% of T4
19
Q

What are the main effects of thyroid hormones?

A
  • Increase carbohydrate metabolism
  • Increase synthesis, mobilisation and degradation of lipids
  • Increase protein synthesis
20
Q

How do thyroid hormones bring about their effects on target cells?

A

Bind to intracellular receptors (nuclear) to influence gene transcription, and therefore protein synthesis.

21
Q

Why are thyroid hormones needed for the CNS?

A

Help with the normal development of the CNS as well as the myelination of nerve fibres.

22
Q

What intracellular effects do thyroid hormones have on cells?

A

Increase the number and size of mitochondrion as well as the number of metabolically important enzymes

23
Q

What aspects of carbohydrate metabolism do thyroid hormones increase?

A
  • Glycogenolysis and glucose uptake
  • Potentiation of the effects of insulin and catecholamines
  • Glucose uptake from GI tract
24
Q

What is Levothyroxine?

A

Drug used to treat hypothyroidism and it can also suppress TSH in the treatment of some thyroid tumours. It is taken either orally or by injection and gets metabolised by the liver through glucuronidation

25
Q

What are some adverse effects of Levothyroxine?

A
  • Palpitations
  • Arrhythmias
  • Diarrhoea
  • Weight loss
26
Q

How does carbimazole work?

A

Inhibit the synthesis of thyroid hormones by preventing the incorporation of Iodine into thyroglobulin so treats hyperthyroidism.

It is a pro-drug so gets converted after absorption into its active form - methimazole. This prevents peroxidase iodinating tyrosine residues on thyroglobulin, meaning less T3 and T4 can be produced.

27
Q

How does propylthiouracil (PTU) work?

A

Treats hyperthyroidism by inhibiting thyroperoxidase which is involved in thyroid hormone synthesis.

Also inhibits thyronine deiodinase to prevent the peripheral conversion of T4 to T3

28
Q

How does potassium perchlorate work?

A

Competes with iodide for the active iodide uptake pump

29
Q

What effect can radioactive iodine have on the thyroid gland?

A

When concentrated on the thyroid gland it causes tissue damage, leading to a reduction in thyroid hormone secretion.

30
Q

What drugs can induce goitre?

A
  • Lithium (used for bipolar disorder and depression)

- Iodides (found in vitamins in some cough remedies)

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