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BRS: Endocrinology > The thyroid gland > Flashcards

The thyroid gland Flashcards

1
Q

Which nerve runs close to the thyroid gland and supplies the vocal chords?

A

left recurrent laryngeal nerve

see diagram 3-5/33

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2
Q

Describe the embryology of the thyroid gland.

A

Midline outpouching from floor of pharynx (originates from base of tongue)

Development of thyroglossal duct

Divides into 2 lobes

Duct disappears leaving foramen caecum

Final position by week 7

Thyroid gland then develops

(see diagram 6-7/33)

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3
Q

Describe the action of TSH on the follicular cell.

A
  • TSH arrives through systemic circulation
  • TSH binds to the TSH receptor on the follicular cell
  • Na+ and I- arrive, and enter through the NA+-I- co-transporter (I- goes into the follicular cell to be transported into the colloid)
  • I- are oxidised (iodination)
  • TSH binding produces thyroglobulin (TG)

TG is a protein - a pro-hormone

TSH stimulation of TSH-R creates TPO (an enzyme) which goes to the colloid

TPO + H2O2 acts as an enzyme for the reaction of iodine + TG -> MIT + DIT

DIT + MIT ( coupling reaction ) also using TPO as enzyme = T3 + T4

T3 + T4 bind to TG to enter back into follicular cell, they are separated by TG by lysosomes and released into bloodstream

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4
Q

Describe the production of T3 and T4 via the mechanism of TSH on the follicular cell.

A
  • TSH arrives through systemic circulation
  • TSH binds to the TSH receptor on the follicular cell
  • Na+ and I- arrive, and enter through the NA+-I- co-transporter (I- goes into the follicular cell to be transported into the colloid)
  • I- are oxidised (iodination)
  • TSH binding produces thyroglobulin (TG)

TG is a protein - a pro-hormone

  • TSH stimulation of TSH-R activates an enzyme, TPO (thyroperoxidase) [this enzyme is important in the production of thyroid hormone]
  • TPO + H2O2 catalyses iodination reactions to produce MIT (monoiodothyronine) and DIT (diiodothyronine)
  • these products join together to give you thyroid hormone
  • T3 + T4 are still bound to TG, enters the cell; in the lysosomes the protein bonds are broken down and thyroid hormone enters the bloodstream

(see diagram 8/33)

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5
Q

What is the active thyroid hormone?

A

T3

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6
Q

What enables TG to be iodinated (in terms of structure)?

A

TG has thyrosine residues (which have an aromatic ring)

there are around 100 residues

of these 100, ~20 can be iodinated

(see diagram 9/33)

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7
Q

What is the main hormone product of the thyroid gland?

A

T4

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8
Q

By which process can you turn T4 into T3?

A

Deiodination

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9
Q

Is T3 the bioactive form?

A

yes

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10
Q

Where is T4 turned into T3?

A

In target tissues

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11
Q

What would happen i you deiodinated T4 in a different position?

A

produce reverse T3 (this is inactive)

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12
Q

Which enzyme enables the conversion of tetraiodothyronine (thyroxine/T4) into its more active metabolite triiodothyronine (T3)?

A

deiodinase enzyme

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13
Q

What is circulating T3 composed of?

A

80% from deiodination of T4

20% from direct thyroidal secretion

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14
Q

What is the role of T3

A

provides almost all of the thyroid hormone activity in target cells

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15
Q

How are T4 and T3 transported in blood?

A

bound to plasma proteins

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16
Q

Which plasma proteins are T3 and T4 bound to?

A

thyroid-binding globulin (TBG) - 70~80%

albumin (10~15%)

prealbumin (transthyretin)

only 0.05% of T4 and 0.5% of T3 is unbound (bioactive components)

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17
Q

Describe the mechanism in which T3 and T4 can effect gene expression.

A

they enter the bloodstream from the thyroid gland and act on target tissues

enter cells through its respective receptors

T4 is deiodinated to T3

T3 enters nucleus with the thyroid responsive element

when T3 binds to thyroid hormone receptor, it can alter gene expression by activating or repressing gene transcription

18
Q

What is thryoid hormone important for?

A

essential for fetal growth and development

especially for the brain and CNS

  • ↑Basal metabolic rate
  • Protein, carbohydrate & fat metabolism
  • Potentiate actions of catecholamines (e.g. tachycardia, lipolysis)
  • Effects on the GI, CNS, Reproductive systems
19
Q

What is cretinism?

A

when a baby is born without either a thyroid gland or any functioning thyroid hormones

looks drowsy, sleepy

this is quite rare now

20
Q

What is the heel-prick test?

A

on day 5 of life

drops of blood removed from heel of feet

compulsory in UK

measurement of TSH

to see whether they have an underactive thyroid

if TSH levels are high, suggest that baby might have congenital hypothyroidism

  • -> if so, can be treated with thryoid hormone replacement
  • -> requires lifelong treatment
21
Q

What is the half lives of T4 and T3?

A

T4: 7-9 days
T3: 2 days

22
Q

How does thyroid hormone effect growth, CNS, BMS, metabolism, cardiovascular?

A

Thyroid hormone binds nuclear receptor -> synthesis of new proteins

Growth: Growth formation, Bone maturation

CNS: maturation of CNS

BMS: ↑Na+/K+ ATPase, ↑O2 consumption, ↑Heat production, ↑BMR

Metabolism: ↑glucose absorption, ↑gylcogenolysis, Gluconeogenesis, ↑lipolysis,
↑protein synthesis & degradation

Cardiovascular: ↑cardiac output

23
Q

Are thyroid disorders more common in men or women, or the same?

A

Women (4:1)

24
Q

Describe the hypothalamo-pituitary-thyroid axis.

A

TRH produced within the hypothalamus

TRH acts on thyrotrophin cells of the anterior pituitary to stimulate the release of TSH

TSH enters systemic circulation and arrives at thyroid gland

production of T3 and T4

but this has negative feedback! (homeostasis)

25
Q

Is an overactive thryoid gland or an underactive thyroid gland more common?

A

the same

overactive = hyperthyroidism
underactive = hypothyroidism
26
Q

What impact can somatostatin have on the production of TSH?

A

It can inhibit TSH production

27
Q

What impact can iodide have on the production of TSH?

A

in large quantities, it can inhibit the production of T3 and T4

if someone has hyperthyroidism, could give them a large amount of potassium iodide to stop any more thyroid hormone being produced from the thyroid

—> this is the Wolff-Chaikoff effect

28
Q

What is primary hypothyroidism?

A

Autoimmune damage to the thyroid

  • Thyroxine levels decline
  • TSH levels climb
29
Q

Why are the most common forms of autoimmune thyroid disease?

A

Hashimoto’s thyroiditis and Graves’ disease.

• Hashimoto’s is usually associated with hypothyroidism

  • Graves’ disease tends to cause hyperthyroidism but sometimes can also cause hypothyroidism

you can also get hypothyroidism as a result of a thyroidectomy

30
Q

What are the symptoms and signs of hypothyroidism?

A
  • Deepening voice
  • Depression and tiredness
  • Cold intolerance
  • Weight gain with reduced appetite
  • Constipation
  • Bradycardia
  • Eventual myxoedema coma
  • fatigue, memory impairment, depression
  • swollen face, swelling of the eye socket, rough, deep or hoarse voice
  • weight gain, constipation
  • dry, rough skin, paresthesia, muscle cramps
  • shaggy hair, hair loss
  • enlarged thyroid gland
  • slowed heart rate, weakness
  • diminished potency and fertility, low sexual desire, cycle disorders
31
Q

What is the proposed mechanism of action of levothyroxine?

A

Normally, you would get T4, produced from the thyroid gland, which is deiodinated to give the active hormone T3.
–
Levothyroxine is almost identical to T4 and can be deiodinated to get T3

32
Q

What is the common dosing for Levothyroxine?

A

Adjusted according to TSH (aim to get into normal range)

Common dose 100 micrograms

Most commonly administered orally

33
Q

What are the potential complications of using Levothyroxine?

A

Minor:
weight loss
headache

Major:
heart attack
rapid HR

34
Q

What is combine thyroid hormone replacement usually composed of?

A

T4 = prohormone, converted by deiodinase action to T3

Combination T4 / T3 – some reported improvement in wellbeing

35
Q

What is Liothyronine used for?

A

It is like T3; however, it is an expensive drug compared to levothyroxine and there is no evidence to show that it works better.

36
Q

What is hyperthyroidism?

A

Thyroid makes too much thyroxine
• Thyroxine levels rise
• TSH levels drop

37
Q

What is hyperthyroidism?

A

Thyroid makes too much thyroxine
• Thyroxine levels rise
• TSH levels drop

38
Q

What can cause hyperthyroidism?

A
  • Graves’ disease – whole gland smoothly enlarged and whole gland overactive
  • Toxic multinodular goitre
  • Solitary toxic nodule
39
Q

What is Graves’ disease?

A

Autoimmune disease

Antibodies bind to and stimulate TSH receptor in the thyroid
• Smooth goitre

Other antibodies bind to muscles behind eye
• Exophthalmos

(see diagram 28-29/33)

40
Q

What are the symptoms and signs of hyperthyroidism?

A
  • Heat intolerance
  • Weight loss with increased appetite
  • Myopathy
  • Mood swings
  • Diarrhoea
  • Tremor of hands
  • Palpitations
  • Sore eyes, goitre
  • nervousness, irritability, insomnia, depression
  • weight loss, strong feeling of hunger, diarrhoea
  • fragile fingernails, shaking hands
  • warm, moist skin, increased body temperature
  • broken hair, hair loss
  • enlarged thyroid gland
  • increased heart rate, arrhythmia, high b.p.
  • muscle cramps, muscle weakness
    (- cycle disorders)

BRS: Endocrinology (10 decks)

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