thyroid testing on Phenobarbital

General questions about tests and procedures for diagnosing canine epilepsy. Please note that we cannot make a diagnosis or interpret specific test results via this forum - please consult your veterinarian for specific advice for your individual pet.

Moderator: Box40Rin

Post Reply
Cathy Whitely
Posts: 1
Joined: Mon Aug 21, 2000 12:01 am
Location: Concord, OH USA

thyroid testing on Phenobarbital

Post by Cathy Whitely » Wed Aug 30, 2000 1:18 pm

What Thyroid testing is most helpful for a dog who's been on Phenobarb for 3 1/2 wks? No previous sign of problem, but now hair falling out, lethargic. I understand that Pheno can alter Thyroid test results, so how are they evaluated?

Ned Patterson DVM
Posts: 209
Joined: Wed Dec 15, 1999 1:01 am
Location: University of Minnesota Veterinary College

Post by Ned Patterson DVM » Thu Aug 31, 2000 8:33 am


Recent studies have shown that in some dogs phenobarbitol can lower levels of total T4 and free T4 the later being the form of thyroid hormone that most accurately reflects thyroid gland status. This happens by the phenobarbitol 'inducing' the liver to metabolize many things more quickly which can include thyroid hormones.

In general total T4 can be low with hypothyroidism or with many other concurrent diseases that have effects on metabolism. The latter is called euthyroid sick - meaning that even though the T4 reading is truly low the thyroid gland function is normal. In general free T4 is a more accurate reflection of what the thyroid gland is doing, and in most cases if it is low the dog is truly hypothyroid. However with the phenobabitols effect on increasing metabolism by the liver in some cases you can have a low free T4 reading in a dog that is not hypothyroid. However if a dog on Phenobarbitol has a low free T4, and clinical signs of low thyroid ( symmetrical hairloss, weight gain, lethargy, cold intolerance etc.) they are likely to have low thyroid function.

Most clinicans recommend a full thyroid panel for dogs with signs of low thyroid that are on phenobarbitol. At a minimum this usually includes - total T4, free T4 by equilibrium dialysis (RIA is not considered as accurate), and TSH (thyroid stimulating hormone). The panel often includes other things such as T3 (the other thyroid hormone), and thyroid auto-antibodies.

If a dog on phenobarbitol has clinical signs of hypothroidism and a low free T4 by equlibrium dialysis - they are more likely to truly have low thyroid function. At that point most clinicians woult treat with thyroid hormone, and if the signs improve significantly the dog is truly hypothyroid. If the signs don't improve and the treatment had brought thyroid readings into the normal range then the dog is probably not hypothyroid and something else is causing the clinical signs.

Posts: 103
Joined: Wed Oct 31, 2012 10:27 pm

Re: thyroid testing on Phenobarbital

Post by daavidfischer » Tue Jul 14, 2015 2:23 am

I think it will be best to stop the Phenobarbital drug for at least two days before doing a thyroid test. Or it won’t actually show any proper result. This is because Phenobarbital affects the decrease in serum thyroxin levels and free thyroxin levels. This will prove to be fatal if she already has thyroid.

remote computer support

Gentle Jacob's mom
Posts: 927
Joined: Thu Mar 04, 2010 10:52 am

Re: thyroid testing on Phenobarbital

Post by Gentle Jacob's mom » Tue Jul 14, 2015 5:39 am

Please don't stop any anti seizure drugs without the guidance and authorization of your vet.


Post Reply