Opposite variations in serum T3 and reverse T3 during propylthiouracil treatment of thyrotoxicosis

Blood samples for determination of serum total and free reverse triiodothyronine (rT3), triiodothyronine (T3) and thyroxine (T4) were obtained daily in 6 previously untreated thyrotoxic patients during periods of propylthiourazil (PTU) (600 mg per day) or methimazol (MMI) (45 mg per day) administration. PTU induced about 60 per cent increase in both total and free serum rT3. This was accompanied by a rapid decrease in serum T3 and a more gradual decline in serum T4. MMI administration to untreated patients was followed by a gradual parallel decrease in rT3, T3 and T4. Turn from PTU to MMI produced a rapid decrease in serum rT3 and increase in serum T3 in all 6 patients. The relative variations in the free and total concentrations of iodothyronines were practically identical. The increase in serum rT3 after PTU is most likely explained either by enhanced deiodination of T4 to rT3 or by an inhibitory effect of PTU on rT3 degradation.