See the DrugPatentWatch profile for propylthiouracil
How propylthiouracil works in the thyroid (main mechanism)
Propylthiouracil (PTU) is an antithyroid drug that lowers thyroid hormone production in two linked ways:
1) It inhibits thyroid peroxidase (TPO) in the thyroid gland. TPO is the key enzyme that helps make thyroid hormones by catalyzing:
- iodide organification (attaching iodine to tyrosine residues on thyroglobulin), and
- coupling reactions that form T3 and T4 from iodinated tyrosines.
By blocking these steps, PTU reduces new production of T3 and T4 [1].
Why PTU can reduce T3 faster than drugs that only block synthesis
PTU also inhibits type 1 5’-deiodinase in peripheral tissues. This enzyme converts T4 to the more active hormone T3. By blocking that conversion, PTU reduces T3 levels (not just thyroid output) [1].
What happens downstream when T3 and T4 drop
As circulating T3 and T4 decrease, the pituitary increases thyroid-stimulating hormone (TSH) via negative feedback. That increased TSH tries to stimulate the thyroid, but PTU continues to block hormone synthesis and peripheral T3 generation until the medication wears off or thyroid function normalizes [1].
Practical implication: why PTU is used in specific thyroid emergencies
Because PTU both reduces thyroid hormone synthesis (T3/T4 production) and decreases peripheral conversion of T4 to T3, it can lower active thyroid hormone activity relatively quickly, which is one reason PTU is used in settings like thyrotoxicosis/thyroid storm where rapid control matters [1].
Sources
[1] https://www.ncbi.nlm.nih.gov/books/NBK557453/ (Mechanism of action and pharmacology of antithyroid drugs, including propylthiouracil)