Q & A: Can L-Thyroxine Treatment Trigger Excessive Shedding And Exfoliation?

I just rechecked a hypothyroid dog that I put on L-thyroxine (Soloxine) about 1 month ago.  The dog's skin is very, very flaky; she is shedding a great deal and is mildly pruritic.  

I came across something in one of my endocrinology books that indicates this may happen when starting thyroid supplementation, but this is the first time I've seen it in my 16 years of general practice.  The skin scrape is negative, and there are no signs of any other skin pathology: there is no evidence of bacterial, yeast, or immune-mediated skin disease. 

So do you think that this excessive shedding and flaking in this dog is related to initiation of the L-T4 therapy?

How frequently do you see this reaction or side effect?

What can I expect this dog's skin to do from here on? How long for this issue to resolve?

My Response:

Hypothyroid dogs occasionally develop severe shedding and flaking during the first 2 to 3 months of L-thyroxine treatment. During this time, the old telogen hairs are all being shed at the same time. With a bit of time on L-T4 replacement, the old shedded hairs are  replaced with anagen hairs, and the skin cycle is restarted.

So I would expect the excessive shedding to resolve within the next 4 to 8 weeks on the L-T4 supplementation. However, you may want to treat the scaling symptomatically with a mild antiseborrheic shampoo and conditioner therapy such as Sebolux or Humilac. Adding some essential fatty acids (EFAs) to the dog's diet may also be of benefit. 

If scaling persists or worsens beyond 8-weeks time, then skin biopsy for a keratinization disorder such as ichthyosis is recommended.

I can't give you an incidence for how often excessive flaking and shedding happens after initiation of L-thyroxine supplementation, but it certainly is not rare. I do see increased flaking more frequently in dogs with Cushing's disease during initial treatment with mitotane or trilostane than I do in dogs with hypothyroidism. 

However, the reason I don't see this problem as often in hypothyroidism may simply be that, as a specialist, I only rarely get to treat and manage hypothyroid dogs. Although hypothyroidism can be a problem to accurately diagnose, it's an relatively easy disorder to treat.