In most dogs with true hypothyroidism, the thyroid gland gradually makes too little hormone. That can affect energy, skin, coat and body weight. The process usually develops over time rather than appearing overnight, and the underlying cause is not something an owner can reliably recognize from symptoms alone.
What happens to the thyroid gland?
The two most common causes are immune-mediated thyroiditis, in which the immune system damages thyroid tissue, and thyroid atrophy, in which functional gland tissue is lost. A few uncommon conditions affect hormone regulation higher up in the body or occur from birth. Breed and inherited susceptibility can play a role, but not every dog in a predisposed breed becomes hypothyroid.
Low blood-test results need context
Many illnesses and medications can temporarily lower total T4 without a dog having true hypothyroidism. Some breeds, especially certain sighthounds, may naturally have lower readings than the general canine reference interval. A veterinarian considers the clinical picture and may measure free T4 and TSH rather than treating one isolated low number.
Symptoms worth discussing
Weight gain without increased feeding, decreased activity, poor coat quality, thinning hair, recurrent skin or ear problems and intolerance of cold can support suspicion. Those signs also have other possible causes. Most diagnosed patients are middle-aged, so a thyroid diagnosis in a puppy deserves especially careful confirmation.
What happens after diagnosis?
Dogs with confirmed disease generally require prescribed levothyroxine and follow-up testing for life. A dose should never be selected from an online chart. Learn about treatment and monitoring and thyroid symptoms.
Veterinary references
- AAHA: Canine hypothyroidism
- AAHA: Diagnostic testing and monitoring
- FDA: Approved treatments for hypothyroidism in dogs
For education only. Thyroid tests and treatment must be interpreted by a veterinarian.
This article is for general education and is not a substitute for care from your own veterinarian. Learn about The Pet Vet and our editorial policy.