Thyroid Drugs Flashcards
In what patients is L-triiodothyronine (T3) contraindicated?
Patients with heart disease
Why is T3 less favorable than T4?
It has a shorter half-life so it must be given multiple times a day, it is more expensive and more difficult to monitor, and carries a greater risk of cardiotoxicity
Discuss the clinical use of L-thyroxin.
(aka T4) is the drug of choice for thyroid hormone replacement therapy
What adverse effects are associated with L-thyroxin (T4)?
Cardiac sx- angina, palpitations
Discuss the clinical use for L-triiodothyronine.
Aka T3, not often used but can be used for hypothyroidism
Describe the mechanism of action of propylthiouracil (PTU) and methimazole (MMI).
They prevent thyroid hormone synthesis by inhibiting TPO (thyroid peroxidase), effectively blocking iodide organification.
What is the clinical use for propylthiouracil (PTU) and methimazole (MMI)?
They are major drugs for thyrotoxicosis
Describe the side effects associated with PTU and MMI, and which is more severe.
Skin rash, joint pain, agranulocytosis. Hepatotoxicity worse with PTU, birth defect risk worse with MMI.
To what drug class do PTU and MMI belong?
Thioamide
Discuss the pharmacokinetics of PTU and MMI.
They have a slow onset of action because they inhibit hormone synthesis, so it takes about 3-4 weeks to deplete thyroid hormone levels.
Describe the mechanism of action of iodide.
It inhibits organification and hormone release, decreases the size of hyperplastic gland. Basically it acts as an autoregulator, so to a point it will increase hormone synthesis but then at a higher level it causes suppression (Wolff-Chaikoff effect)
What is the clinical use for iodide?
Previously used for hyperthyroidism. Now it is used in combo w/ PTU and a beta-blocker in thyroid storm, only short-term therapy.
What treatment combination is used during a thyroid storm?
Iodide, PTU, and a beta-blocker.
When is iodide contraindicated?
In pregnancy- it can cause fetal goiters.
Describe the mechanism of action of I^131 (radioactive iodide)
It rapidly concentrates within thyroid follicle cells, where beta particles selectively destroy the gland without injury to adjacent cells
