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SPR L16 Thyroid Drugs Flashcards

1
Q

Learning Outcomes

A
  • Overview of thyroid hormone synthesis
  • Review thyroid receptor mechanism of action
  • Hyperthyroidism and treatment options
  • Hypothyroidism treatment
  • Drugs that effect thyroid hormone synthesis
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2
Q

Thyroid Hormone Synthesis

  1. What stimulates gene transcription for the carrier?
  2. Outline thyroid hormone synthesis
A
  1. TSH
  2. Uptake of iodine into follicle cell, iodination and coupling, storage in colloid and endocytosis and secretion back into plasma of T3 (Triiodothyronine) and T4 (Thyroxine)
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3
Q

What regulates thyroid?

  1. What acts upon the hypothalamus?
  2. What does this bring about?
  3. What acts negatively on the Ant Pit?
  4. What acts +vely on the Thyroid gland?
  5. What acts -vely ont eh thyroid gland?
A
  1. Cold, Trauma, Stress
  2. Release of TRH to Ant Pit, then TSH (thyrotrophin) to Thyroid Gland, T3 and T4 (may be changed into T3) release
  3. Somatostatin
  4. Iodine (physiological levels)
  5. Iodine (excess), thioureylenes, excess of exogenous iodide, 131I
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4
Q

Thyroid Hormone

Mechanism of Action

  1. What is action mainly through?
  2. Give examples of Superfamily 4 intracellular receptors
  3. What is the structure of these receptors?
  4. Generally, what action does binding to these Rs bring about?
A
  1. Thyroid Nuclear Receptor TR (Superfamily 4)
  2. oestrogen receptor (antagonist = tamoxifen)

androgen receptor

thyroid hormone receptor

vitamin D receptor

glucocorticosteroid hormone receptor

mineralocorticosteroid hormone receptor

  1. COOH - Agonist Binding Site - DNA Binding Domain with 2 ‘Zinc Fingers’ - Regulatory Domain. SEE PICTURE
  2. enhance or suppress gene expression
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5
Q

Mechanism of action of Type 4 receptors

(enhance or suppress gene expression)

Describe the mechanisms of Type 4 Rs

A
  • agonist crosses cell membrane and binds to receptor
  • zinc finger domain opens up and binds to hormone responsive elements located upstream from promotor region of gene
  • ↑ ↓ activity of RNA polymerase
  • ↑ ↓ gene transcription
  • ↑ ↓ translation of mRNA
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6
Q

Mechanism of Action of Thyroid Hormone

How does it act?

A
  • Mainly through Thyroid Nuclear Receptor TR (Superfamily 4)
  • T3-TR bound stimulates nuclear transcription of target genes, increasing mRNA and protein synthesis
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7
Q

Thyroid Hormone - Effects

  1. Which is more potent, T3 or T4?
  2. What are the effects of thryoid hormone?
A
  1. T3 = x5 T4
  2. Increase in basal metabolic rate (Heat, Oxygen consumption), Increased heart rate – potential for dysrhythmias (AF), Potentiate Growth Hormone effects / CNS development
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8
Q

What are the signs of Hyperthyroidism / Thyrotoxicosis?

A
  • Nervous / edgy
  • Shaky
  • Palpitations
  • Heat intolerance
  • Increased appetite / decreased weight
  • Almost manic levels of energy
  • Tremor
  • Hyper-reflexia
  • Sweaty / warm to touch
  • Tachycardic
  • May be signs of the type of Hyperthyroidism
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9
Q

Treatment of Hyperthyroidism

Generally, what are the treatment options?

A
  • Pharmacologically (surgically if tracheal compression)
    • Options
      • Block
      • Block and replace
      • Radioactive Iodine
      • Surgery
  • Symptomatic control
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10
Q

Treatment of Hyperthyroidism

Block - Thioureylenes

  1. Give examples of drugs used to block
  2. How do they act?
A
  1. Carbimazole, Propylthiouracil
  2. Inhibit iodination of Tyr residues in TG

? Inhibit thyroperoxidase?

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11
Q

Treatment of Hyperthyroidism

Block - Thioureylenes

​​Carbimazole

  1. How are they given?
  2. What is this drug converted to?
  3. What is their distribution and T1/2?
  4. What effects does it have?
A
  1. Given orally
  2. Carbimazole is converted to methimazole
  3. Widely distributed, T ½ = 6-15 hours
  4. 90% inhibition of iodination of TG within 12 hours. Causes a gradual reduction in signs and symptoms of hyperthyroidism (4 weeks)
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12
Q

Treatment of Hyperthyroidism

Block - Thioureylenes

​​Carbimazole

  1. How should this be started?
  2. What should be done afterwards?
  3. How long should the maintenenance level be kept for?
  4. What if hyperthyroidism recurs?
A
  1. Initially Carbimazole 15-40mg od for 4-8 weeks (propylthiouracil if intolerant)
  2. Gradually reduce Rx to maintenance 5-15mg o
  3. Rx at maintenance level for 18 months, At that time discuss discontinuing Rx - Some will remain in remission
  4. they will need Radioactive Iodine Rx
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13
Q

Treatment of Hyperthyroidism

Block - Thioureylenes

​​Carbimazole

  1. What are the common side effects?
  2. Name a rare side effect?
    1. What should be reported?
  3. What needs to be given in pregnancy?
A
  1. Common – rash / pruritus / nausea / less hyper
  2. Rarely: Agranulocytosis (0.03%)
    1. Patient should report symptoms and signs of bone marrow suppression / infection (esp sore throat)
  3. Pregnancy – lowest effective dose carbimazole as it crosses the placenta so may cause goitre / hypoT4
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14
Q

Treatment of Hyperthyroidism

Symptomatic Rx

  1. What drug is used?
  2. What does it give?
  3. What does it inhibit?
  4. When is it used?
  5. Give an example dose
  6. What is it especially good to improve?
A
  1. Propranolol – non-selective beta blocker
  2. Rapid relief of thyrotoxic symptoms
  3. Also inhibits T4 to T3 conversion in the peripheral tissues
  4. Used as an adjunct in early few weeks of Rx
  5. E.g. 40mg tds orally
  6. Especially good to improve palpitations / arrhythmias / tremor / agitation
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15
Q

Treatment of Hyperthyroidism

Block and Replace

What drugs are used?

How long are they given for?

When is this NOT suitable?

A

? Less TSH therefore less goitre?

Carbimazole 40mg-60mg od

PLUS

Levothyroxine 50 – 150 microgram od

Again given for 18 months

IF PREGNANT

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17
Q

Treatment of Hyperthyroidism

Radioactive Iodine – 131I

  1. How is this given?
  2. What occurs?
  3. T1/2?
  4. What does it lead to?
  5. What will the patient therefore need?
A
  1. Given orally as one dose
  2. Rapidly and selectively taken up in the Thyroid, Emits short range beta particle radiation damaging only the thyroid follicle cells
  3. 8 days (gone in 2/12)
  4. Leads to iatrogenic hypothyroidism after about 4/12
  5. Therefore patient will need lifelong T4 Rx
18
Q

Hypothyroidism

What are the symptoms?

A
  • ‘Foot off the accelerator’
  • Feel cold, tired, sluggish
  • Slow comprehension
  • Delayed reflexes
  • Weight gain, despite poor appetite
  • Dry thickened skin
  • Loss of hair (esp outer 1/3 of eyebrow)
19
Q

Hypothyroid - Treatment

  1. What drug is used?
  2. What is the dose?
  3. How is it introduced?
  4. When are there side effects?
A
  1. Levothyroxine
  2. 50-150 micrograms od (Adjusted to symptoms and biochemistry)
  3. Introduce slowly 25-50 micrograms increased every 4 weeks especially if in the elderly
  4. only if in excess

Note individually adjusted in 25’s

20
Q

Drugs that affect thyroid hormone synthesis

Give examples

A
  • Thioureylenes
  • Iodides
    • Lugols iodine
    • Amiodarone
    • Iodinated contrast media
  • Lithium
    • Decreases thyroid function
    • Rarely significantly
    • Check TSH 6-12 months
21
Q

Drugs that affect thyroid hormone synthesis

Lugol’s Iodine

  1. What is this rarely used?
  2. What does it bring about?
A
  1. poorly tolerated
  2. Pharmacological levels of iodine: Inhibit hormone release. Short-term control of thyrotoxicosis pre-op. Advantage of reducing the vascularity and fragility of the gland.
22
Q

Drugs that affect thyroid hormone synthesis

Amiodarone (antiarrhythmic)

What effects than this drug have?

A
  • Due to high tissue binding and Iodine moieties
  • Can cause Hypo or Hyperthyroidism in approx 5% of patients on this drug
  • Should check and monitor TFTs before and during Rx
23
Q

Summary

A

Hyperthyroidism

  • Carbimazole (or propylthiouracil) blocks TG iodination, acts fast but effects seen in 4 weeks
  • Warn re: agranulocytosis
  • Propranolol treats symptoms at the early stages
  • Give 18 months maintenance carbimazole then stop
  • Radioactive 131I for patients who relapse

Hypothyroidism

  • just give the hormone low and slow

Pharmacology (26 decks)

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