Carbimazole Mechanism of Action Flashcards
What is the role of thyroid peroxidase (TPO) in iodide oxidation?
TPO catalyzes the oxidation of iodide (I⁻) to iodine (I₂), an essential step in thyroid hormone synthesis.
How does TPO participate in the iodination of tyrosine residues?
TPO iodinates tyrosine residues on thyroglobulin to form MIT (monoiodotyrosine) and DIT (diiodotyrosine).
What role does TPO play in the coupling reaction during thyroid hormone synthesis?
TPO catalyzes the coupling of two DITs to form T4 (thyroxine) or one MIT and one DIT to form T3 (triiodothyronine).
Where in the thyroid gland does TPO function primarily?
At the apical membrane of follicular cells, where it faces the colloid space.
What would be the effect of TPO inhibition on thyroid hormone levels?
Inhibition of TPO leads to decreased synthesis of T3 and T4, resulting in reduced thyroid hormone levels.
What is the primary mechanism of action of carbimazole?
Inhibition of thyroid hormone synthesis by blocking thyroid peroxidase (TPO).
Is carbimazole a prodrug? If yes, what is its active form?
Yes, it is a prodrug. It is converted into methimazole, the active form.
Which enzyme does carbimazole (via methimazole) inhibit in the thyroid gland?
Thyroid peroxidase (TPO).
What process involving iodine does carbimazole inhibit?
The iodination of tyrosine residues on thyroglobulin.
How does carbimazole affect the coupling of iodotyrosines?
It inhibits the coupling of MIT (monoiodotyrosine) and DIT (diiodotyrosine) to form T3 and T4.
Does carbimazole reduce circulating levels of pre-formed T3 and T4?
No, it only prevents new synthesis; it does not affect already circulating hormones.
How long does it take for carbimazole’s effects to become clinically apparent?
Typically 1–2 weeks due to the depletion of stored hormones.
What role does carbimazole play in the negative feedback loop of the thyroid axis?
By reducing T3/T4 synthesis, it allows TSH levels to normalize, helping restore balance.
Why is carbimazole ineffective in thyrotoxicosis factitia?
Because it does not affect exogenous thyroid hormone already in circulation.
What is the ultimate goal of carbimazole therapy in hyperthyroidism?
To reduce excess thyroid hormone production and restore euthyroid state.
