Dynamics of Inhibition of Iodothyronine Deiodination during Propylthiouracil Treatment of Thyrotoxicosis

Summary

Serum 3,3′,5′-triiodothyronine (rT₃), 3,5,3′-triiodothyronine (T₃), thyroxine (T₄) and propylthiouracil (PTU) were measured before and at short intervals for 8 hours after oral administration of 200 mg PTU to six patients with untreated thyrotoxicosis. The study was repeated on the 4th day of treatment with 200 mg PTU every 8 hours. Six other patients with untreated thyrotoxicosis were studied after a single administration of 800 mg PTU.

The results indicated that PTU inhibition of T₄ deiodination to T₃, evaluated by the fall in serum T₃, was of the same duration as the PTU inhibition of rT₃ deiodination, evaluated by the increase in serum rT₃. After 200 mg PTU inhibition was maximal for only a few hours, and there was no cumulative effect of PTU during the first four days of treatment, when 200 mg PTU was given every 8 hours. After 800 mg PTU the full effect was maintained for the 9 hour period studied, after which serum T₃ had fallen to 65 ± 2 % of the pretreatment level (mean ± SE). Thus, to obtain a permanent full effect of PTU on iodothyronine deiodination during the treatment of thyrotoxicosis it is necessary to use large doses or frequent administration.

Key-Words:

Thyrotoxicosis - Propylthiouracil - T ₄ Deiodination