Thyrosyn for Dogs

Oval orange pill.
Thyrosyn comes in many different strengths that are differentiated by their colors.

Getty Images/Nikolai Mentuk

Thyrosyn is the brand name of a medication that is used for treating an underactive thyroid gland in dogs. This drug, and generic versions, are only available through prescriptions from your dog's veterinarian.

Knowing how this drug works and what potential side effects you should watch for can help you better manage this disease if your dog is diagnosed with hypothyroidism.

What Does Thyrosyn for Dogs Do?

Thyrosyn contains the drug levothyroxine sodium and replaces the thyroid hormone that a dog with hypothyroidism cannot produce on its own. It is typically dosed at about 0.1 mg/10 lbs. of body weight and is absorbed rapidly in the gastrointestinal tract after the oval pill is taken by mouth.

The dosage is calculated based on the weight of the dog initially but may need to be adjusted depending on the results of ongoing blood tests. T-4 and T-3 blood levels are often checked every four weeks after a dog is diagnosed with hypothyroidism. This is because Thyrosyn directly affects these levels and provides an indication of how well the disease is being managed. Once the blood levels are maintained at an adequate level, testing may only be needed every six to twelve months.

Thyrosyn comes in a variety of colors and corresponding dosage strengths to ensure the correct pill is being given to your dog. The 0.1 mg pills are yellow, 0.2 mg are red, 0.3 mg are green, 0.4 mg are maroon, 0.5 mg are white, 0.6 mg are purple, 0.7 mg are orange, 0.8 mg are blue, and 1 g pills are beige.

Various generic levothyroxine sodium products and other brands are also available to increase thyroid hormone levels in dogs. These products include L-Thyroxine, Thyro-L, Thyro-Tabs, Soloxine, Levocrine, Levotabs, Synthroid, and others. Human thyroid hormone products are not recommended for use in dogs since humans take much lower dosages.

Thyrosyn does the opposite of what methimazole or thiamazole does. These drugs are also known by the brand names Tapazole and Northyx and are used for treating hyperthyroidism, a condition more commonly seen in cats.

Diseases Thyrosyn Can Treat

Thyrosyn treats hypothyroidism which is a result of an underactive thyroid gland. This gland is located in the neck of a dog and hypothyroidism occurs if it shrinks, becomes inflamed, or if cancer develops in or near the gland.

Inadequate amounts of thyroid hormone are produced by shrunken or inflamed thyroid glands so if these hormone levels are too low, Thyrosyn will increase them.

Hypothyroidism is a life long disease so since there is no cure for it, dogs that are taking Thyrosyn will need to be on it long term.

Side Effects of Thyrosyn Use

Thyrosyn is considered to be a safe drug and side effects are not associated with administering this product to dogs at recommended amounts. But, if a dog consumes too many Thyrosyn tablets or takes Thyrosyn without having hypothyroidism, thyrotoxicosis is likely to occur. This is when an excessive amount of the thyroid hormone is in the body.

Symptoms of thyrotoxicosis include panting, nervousness, hyperactivity, increased heart rate, increased drinking and urination, vomiting, diarrhea, and weight loss.

If your dog consumes too many Thyrosyn tablets or is showing signs of thyrotoxicosis, you should contact your veterinarian immediately.

Considerations Before Using Thyrosyn in Dogs

Prior to administering Thyrosyn, your dog should be diagnosed with hypothyroidism by having blood tests performed to confirm the thyroid gland is not producing enough thyroid hormone. A dog with euthyroidism, defined as having funtioning thyroid glands and normal thyroid hormone levels, should not take Thyrosyn.

Dogs with hypertension (high blood pressure), diagnosed heart disease, uncorrected Addison's disease are also not recommended to take Thyrosyn.

Testing to ensure Thyrosyn is safe to be administered in pregnant and nursing dogs has not been done.

Storage of Thyrosyn

Storage of medications is very important. If stored incorrectly, many products, including Thyrosyn, won't be as effective in helping your dog.

Thyrosyn should be stored away from light and at room temperature below 104 degrees. Storing it in a cabinet and using the packaging the product came in from the manufacturer, or an amber, blue, or green pill vial from your veterinarian, will protect Thyrosyn from harmful light and elevated temperatures.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the their health history, and can make the best recommendations.