Hypothyroidism in Dogs

In dogs, hypothyroidism is most often caused by primary disease of the thyroid gland itself. The two main causes are lymphocytic thyroiditis, an immune-mediated process that damages thyroid tissue, and idiopathic thyroid gland atrophy, where normal thyroid tissue is replaced over time. Together, these account for the large majority of cases. Much rarer causes include thyroid cancer, pituitary disease, congenital problems, or trauma. Because many other illnesses and some medications can lower thyroid hormone test results without true thyroid failure, veterinarians also consider euthyroid sick syndrome before making a diagnosis.

Risk factors and predispositions include: • Middle age, most commonly about 4 to 10 years old • Possible increased frequency in spayed females and neutered males • Breed predispositions reported in dogs such as Golden Retrievers, Doberman Pinschers, Cocker Spaniels, Irish Setters, Dachshunds, Boxers, and some other medium-to-large breeds • Heritable tendencies in some lines, especially with autoimmune thyroiditis • Concurrent illness that can complicate thyroid testing and mimic disease

Diagnosis should not be based on a single low thyroid number alone. Your veterinarian will start with a full history, physical exam, and routine lab work, then interpret thyroid testing in the context of your dog's clinical signs.

What your vet may do: • Review signs such as lethargy, weight gain, skin disease, ear infections, cold intolerance, or coat changes • Run baseline blood work and a urinalysis to look for supportive changes and rule out other illnesses • Check a total T4 (TT4) as an initial screening test • Confirm suspicious results with more specific testing, often free T4 by equilibrium dialysis and canine TSH • Consider cholesterol and triglyceride changes, which can support the diagnosis but are not specific • In selected cases, use additional thyroid panels, thyroglobulin autoantibody testing, thyroid ultrasound, or other advanced testing

At the appointment, expect your vet to discuss why hypothyroidism is often overdiagnosed and why other diseases can cause low thyroid test results. Careful interpretation matters because treating the wrong condition can delay the correct diagnosis.

Canine hypothyroidism is treatable but usually not curable. Most dogs are managed with lifelong thyroid hormone replacement prescribed by a veterinarian, typically levothyroxine. Your vet will recheck thyroid levels and clinical response after starting therapy and will adjust the plan based on follow-up testing.

Treatment and management may include: • Prescription thyroid hormone replacement for life • Regular follow-up blood testing to confirm an appropriate response • Treatment of secondary problems such as skin infections or ear infections when present • Weight management, routine exercise, and nutrition support if weight gain has occurred • Ongoing monitoring for improvement in energy, skin, coat quality, and body weight

Many dogs improve noticeably in attitude and energy within a few weeks, while skin and coat recovery may take several months. If a dog does not improve as expected, your vet may reassess the diagnosis or look for another underlying problem.