My dog was on soloxine for 3 years.She was hypothyroid,had cushing's and an enlarged esophagus.I had to put her down 3 weeks ago because of complications from her esophagus problem.According to my vet,Soloxine is a safe,longterm medication.
I have a beagle female who has been on low level Soloxine therapy for the past 3 years. Borderline low normal thyroid levels coupled with some autoimmune problems were the reason for treatment. It's been relatively effective.