Warfarin Institute of America
Dedicated To Your Health
PROPYLTHIOURACIL INTERACTIONS WITH WARFARIN
Also known as PTU.
DO NOT UNDERESTIMATE THE SERIOUSNESS OF THIS INTERACTION . THIS IS NOT A VERY COMMONLY PRESCRIBED MEDICATION, SO NOT MUCH IS KNOWN ABOUT THIS INTERACTION. I HAVE ONLY SEEN IT ONCE BUT THAT WAS ENOUGH TO CONVINCE ME OF HOW MUCH DAMAGE THIS INTERACTION CAN DO.
The following is a case study of how I managed a woman who experienced this interactuion. Note that it took seven months to get her INR back under control.
2-1 Started on propylthiouracil (PTU). She neglected to lower her warfarin dose to 30 mg/wk as planned.
2-7 Took PTU 50 mg/day and warfarin 35 mg/wk. INR = 4.6 Warfarin reduced to 30 mg/wk.
2-21 PTU dose increased to 300 mg/day. Took warfarin 30 mg/wk. INR = 2.8. Continue warfarin 30 mg/wk.
3-12 She awoke with what she thought was migraine headache. Over the next few hours, this progressed to a slight, left-sided paralysis and numbness. She described not being able to feel the stick of the IV being inserted in her arm but felt the sensation of tape being pulled off the hairs on her arm. Her CT scan on admission was essentially negative for any new disease. A bilateral carotid artery ultrasound showed normal flow and no significant plaque. She had taken warfarin 30 mg/wk. Her INR was 1.2. The next day, an enhanced MRI showed multiple, tiny areas of acute infarction involving both basal ganglia and the parenchyma of both parietal lobes. There was a suspicion of trace amounts of subarachanoid hemorrhage or possible petechial hemorrhage. These were not present on the previous day's CT scan. In plain English, she had several small strokes and a small amount of bleeding into the brain.
3-20 She made a good recovery and was discharged from the hospital. The PTU was stopped and methimazole was started. In addition, her diuretic dose was increased, as was her potassium dose. She took either warfarin 47.5 or 2.5 mg/wk. INR = 4.6. It appears that PTU and methimazole do not have an equal effect on the INR. Hold warfarin one day, then continue warfarin 7.5 mg/day. This may seem illogical, but remember that this patient just had a stroke. There is more evidence to support a stroke being caused by a low INR than there is for a bleed being caused by an INR under 5.0. Also, her INR was to be checked again in three days.
3-23 Continues on methimazole. Took warfarin 45 mg/wk. INR = 4.5. Decrease warfarin to 40 mg/wk.
3-26 Took methimazole and warfarin 40 mg/wk. INR = 4.6. Reduce warfarin to 37.5 mg/wk.
3-30 Her thyroid tested normal during the past week. Continues on methimazole. She has a toothache and was started on penicillin. She reports bruising easily. INR = 6.6. This may be due to eating less and therefore taking in less vitamin K. This was a Friday and she had already taken her warfarin dose for that day. Hold warfarin on Saturday. On Sunday, take warfarin 5 mg. On Monday, take no warfarin until the INR has been checked.
4-2 The bruise reported at the last visit is improving. Took warfarin 32.5 mg/wk. INR = 3.2. Continue warfarin 32.5 mg/wk.
4-6 She may have her tooth extracted in three days. Took warfarin 32.5 mg/wk. INR = 3.9. If her tooth continues to hurt, she will hold warfarin for a possible extraction. If the tooth does not hurt, she will continue the warfarin.
4-13 She did not have a tooth extraction. She took warfarin 30 mg/wk. INR = 2.7. Continue warfarin 325 mg/wk.
5-4 She doubled her dose of methimazole. Took warfarin 30 mg/wk. INR = 2.7. Continue warfarin 30 mg/wk.
In this interval she had several INR measurements which were in the desired range with no medication changes.
7-16 Methimazole dose reduced. Took warfarin 30 mg/wk. INR = 4.0. Reduce warfarin to 25 mg/wk.
8-22 Stopped methimazole yesterday. Took warfarin 27.5 mg/wk. INR = 2,2. The INR is too low, but it should increase as the methimazole wears off. Take warfarin 2.5 mg today, and then continue warfarin 27 mg/wk.
8-29 Took warfarin 27.5 mg/wk. INR = 4.4. The INR did go up as the methimazole wore off. Decrease warfarin to 25 mg/wk.
9-5 Took warfarin 25 mg/wk. INR = 2.4. Increase warfarin to 27,5 mg/wk.
This is a good example of how the INR mirrors the thyroid activity. When the thyroid is overactive or a thyroid replacement medication is increased, the INR will increase. The same is true if an anti-thyroid medication is decreased, the INR will increase. The opposite is also true. If the thyroid is underactive, or a thyroid replacement medication decreased, the INR will decrease. The same is true with an anti-thyroid medication, it is increased, the INR will decrease.
For a further discussion of the mechanism of this interaction, click here.
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Last updated November 15, 2004