Thyroid & Antithyroid

Question 0

What is the mechanism of action of levothyroxine? When is peak therapeutic effect reached?

Answer 0

Converted to T3 in peripheral tissues
Longer half life than Lioythyronine (T3)
Takes 3-4 weeks to reach peak effect

Question 1

What are the 3 synthetic thyroid replacement agents. Which is T3, which is T4 and which is a mixture? Which one is preferred?

Answer 1

Levothyroxine Sodium (T4) *preferred
Liothyronine Sodium (T3)
Liotrix (mixture T3/T4 in 1:4 ratio)

Question 2

Which synthetic thyroid agent is given in acute emergency conditions (myxedema coma)?

Answer 2

Lioythyronine (T3)
Rapid-acting and very potent
[peak levels in 2-4hrs; 4x as potent as levothyroxine]

Question 3

What are some side effects of Liotrix and those in its class?

Answer 3

Heart palpitations
Nervousness
Heat intolerance
Excessive sweating
Insomnia
Tremors
Frequent bowel movements
Excessive weight loss
[SEs similar to hyperthyroidism]

Question 4

What is Cretinism?

Answer 4

Infancy/childhood hypothyroidism (iodine deficiency, symporter gene mutation)
Causes mental retardation and dwarfism

Question 6

What is the most common cause of hyperthyroidism? What happens? What are 3 other causes?

Answer 6

• Anti-TSH receptor Ab which activates thyrotropin receptor on thyroid cells
• affects women between ages 20-40
• other causes: nodular goiter, thyroiditis, thyroid cancer

Question 7

What are some precautions to Levothyroxine use?

Answer 7

“Start low, go slow” in patients with angina pectoris, CAD, and elderly;
requires periodic monitoring and adjustment (especially during pregnancy for fetal brain development);
not effective in Chronic Fatigue Syndrome

Question 8

What are 3 antithyroid agents? Mechanism?

Answer 8

Propylthiouracil
Methimazole (active metabolite of Carbimazole)
Carbimazole [Europe]
-Thyroid peroxidase inhibitors
–>Inhibit organification of iodide and coupling of iodotyrosine [inhibits thyroid hormone synthesis]; also reduces peripheral deiodination of T4 to T3 (propylthiouracil)

Question 9

Although very rare, what are 3 severe side effects of Carbimazole and those in its class? Which one is preferred for use during pregnancy?

Answer 9

(thyroid peroxidase inhibitor)
Agranulocytosis (seen more with Propylthiouracil)
Hepatitis
Lupus-like syndrome
–Propylthiouracil preferred during pregnancy since less likely to cross placenta

Question 10

What is the treatment of choice for relapsed hyperthyroidism after drug therapy? Mechanism? Contraindications?

Answer 10

Radioactive Iodine (131-I)

• Radioactive agent that emits beta particles and gamma rays that destroy thyroid cells (effects may take weeks)
• Contraindicated in pregnant/breast feeding, patients under 10; monitor serum thyroid hormones post-treatment every 2-3 months for first year

Question 11

What is the purpose of beta-blocker use in hyperthyroidism? Contraindications?

Answer 11

• Reduce sympathetic tone (tremor, anxiety, palpitations)
• Only used as adjunct if symptoms moderate-severe and should be discontinued once euthyroid
• contraindicated if risk of heart failure