Thyroxine, L-Thyroxine, Levothyroxine, Thyro-tabs: They are all different names for a common dog medication. It is a synthetic form of a hormone normally made by your dog's thyroid gland and is used to treat hypothyroidism.
What Does Thyroxine Do?
Thyroxine (generic drug name being Levothyroxine) is a manufactured medicinal compound. It is prescribed to replace the endogenous thyroxine your hypothyroid dog's thyroid gland should be making, but for some reason, just isn't. When there are adequate concentrations of thyroxine in your dog's blood stream, the activity of cells that are responsible for growth, tissue development, maintenance of brain function, regulation of body temperature, and other cellular processes is increased.
The oral medication is given every 12 to 24 hours, as the drug clears your dog's body after 24 hours. If your dog has any concurrent liver or kidney disease, it may take a bit longer for your dog's body to clear the drug. The actual amount of medication your dog's body absorbs is also dependent on whether or not they eat a meal with the medication or if the medication is given on an empty stomach. So be consistent in how you give your dog his thyroid medication–either always with his meals or always without.
Diseases and Illnesses that Thyroxine Can Treat
Thyroxine is a vitally essential medication if your dog has hypothyroidism. Hypothyroidism is a disease process associated with your dog's endocrine system. The endocrine system is responsible for creating hormones that control growth and development, metabolism, organs, and reproduction. Other endocrine diseases include diabetes, Cushing's disease, and Addison's disease. Unfortunately, like many other endocrine illness, hypothyroidism is not something that can be cured with modern veterinary medicine. Rather, the symptoms of the disease are managed with daily medication.
The most common symptoms seen with hypothyroidism, the ones that daily oral Thyroxine medication manages, include hair loss/thinning hair coat, a dull or dry and brittle coat, recurrent skin infections, weight gain, reduced activity, heat-seeking and intolerance of the cold, anemia, and high cholesterol. Typically, you will start to see improvement in your dog's activity level just a few weeks after starting the Thyroxine. Hair regrowth can take a bit longer, sometimes up to four months.
Side Effects of Thyroxine Use
When given the appropriate dose, the side effects of oral Thyroxine are rare. Skin reactions can occur, but they are quite infrequent. If given at too high of a dose, your dog may experience thyrotoxicosis. Signs of this may include increased heart rate, appetite, drinking, urination, excitability, panting, and nervousness. Thyrotoxicosis can also occur if your dog has a concurrent illness that effects your dog's ability to break down the medication (such as liver disease or kidney disease), or if your dog is just overly sensitive to Thyroxine.
To monitor your dog's thyroid level and ensure that are on the correct dose, your vet will want to check your dog's blood work every six to 12 months. If your dog recently started taking Thyroxine or if they had a dosage change, your vet will want to check blood work four to six weeks after starting the new dose. Thyroxine levels in your dog's blood will peak four to six hours after the pill is given. So for the most accurate reading, your dog should have his monitoring blood drawn 4 to 6 hours after you give them their pill.
According to the Food & Drug Administration (FDA), side effects of Thyroxine can also include ineffectiveness of the medication. That is, if it just isn't doing what it's supposed to be doing, or if you yourself have a reaction simply from handling the medication. Speak to your vet if you note either of these adverse effects.
Considerations Before Using Thyroxine for Dogs
If your dog has concurrent heart problems, Addison's disease, or Cushing's disease, speak to your vet before starting your dog on Thyroxine. The dosages of heart medications or medications your dog takes for his Addison's or Cushing's disease may need to be adjusted once your dog is started on Thyroxine. Certain heart medications can cause thyroid levels to be low to begin with. Thyroxine can also interact with medications such as oral antacids (such as pepcid), corticosteroids (such as prednisone, prednisolone, dexamethasone, and methylprednisolone), tricyclic antidepressants (such as amitriptyline) phenobarbitol, sucralfate, and sertraline. So mention these to your vet if your dog is taking any.
Certain breeds of sighthounds (such as greyhounds, whippets, and Irish wolfhounds) can also have thyroid levels that are normally much lower than other dog breeds. Your vet will keep this in mind when checking your sighthound's thyroid level.
Thyroxine was approved for use in dogs in 2016. Since then, many vets have seen success in managing the symptoms of hypothyroidism with it. If you think your dog may be suffering from hypothyroidism, speak to your vet about checking for the disease and possibly starting them on Thyroxine.
Levothyroxine. VCA Hospitals.
Hypothyroidism in Dogs. VCA Hospitals.
Hypothyroidism in Dogs—There are FDA-Approved Drugs to Treat It. U.S. Food and Drug Administration.
Shiel, Robert E et al. Assessment of criteria used by veterinary practitioners to diagnose hypothyroidism in sighthounds and investigation of serum thyroid hormone concentrations in healthy Salukis. Journal of the American Veterinary Medical Association vol. 236,3 (2010): 302-8. doi:10.2460/javma.236.3.302