Hypothyroidism – One of the Most Common Diseases in Dogs

Certain breeds are predisposed to hypothyroidism

This week I will discuss an important topic that seems to cause much confusion with pet parents; hopefully, understanding the letters and names associated with this condition will help this information make sense. This is one of those topics where becoming familiar with the disease itself as well as some of the words that the doctors use could actually keep your pet safe. I find that keeping things visual and simple is the best way for me to understand and retain a particular subject. I will start with a few basic definitions and unlike other topics which can be tackled directly in paragraph form, I will attempt to keep things as visual and clear as possible.


 a prefix used when describing something as being less than or lower


 a prefix used when describing something as being greater than or more


 overactive thyroid


 underactive thyroid

 Thyroid Gland

 generally found in the neck and its function is to produce T3 and T4


 Triiodothyronine, produced by the thyroid gland


 Thyroxine, produced by the thyroid gland and along with T3 regulates many important body functions such as metabolism

The thyroid gland has the role of converting iodine into thyroxine (T4) and triiodothyronine  (T3). The T4 and T3 circulate throughout the body and affect almost every single organ. Thyroxine regulates growth, metabolism, immune function and heart function. Thyroid stimulating hormone (TSH) from the pituitary gland is what signals the thyroid gland to secrete thyroxine. Thyroid disease is common in both dogs and cats but the type of thyroid disorder usually varies depending on the species. In dogs, thyroid disease is usually due to lowered secretion of thyroid hormones (hypothyroidism) and in cats it is usually due to an overproduction of thyroid hormones (hyperthyroidism). Hyperthyroidism is quite rare in dogs; similarly, hypothyroidism is rare in cats. Because of this, the medications used to treat these conditions are usually quite different when treating dogs or cats.


More common in cats


More common in dogs

Let’s start with hypothyroidism in dogs. Hypothyroidism happens when the thyroid gland stops producing thyroid hormone. Most cases of hypothyroidism in dogs happen because the thyroid gland gets harmed or destroyed by the immune system itself. Sometimes hypothyroidism can be caused because TSH is not being secreted in enough quantity.

Predisposing Factors to hypothyroidism

  • Mid to large size breeds:  Cocker Spaniels, Golden Retrievers, Labrador Retrievers, Dachshunds, Doberman Pinchers, and Irish Setters
  • Middle aged to older dogs
  • Neutered

Signs and Symptoms
A dog can have hypothyroidism for many years without showing any signs or symptoms. The development of signs and symptoms is a gradual process and generally includes many organ systems. This process can occur over months to years. Early detection is important to prevent complications.
Complications of the skin are usually the most common:

  • Thinning or loss of hair
  • Dull hair coat
  • Excessive shedding
  • Poor wound healing

Central Nervous System:

  • Mental dullness
  • Mood swings
  • Disorientation
  • Lameness


  • Diarrhea
  • Vomiting

Additional Signs and Symptoms:

  • Weight gain or obesity (decreased metabolism)
  • Reduced activity and seeming week or lethargic
  • Reduced ability to tolerate cold climates
  • Slow heart rate
  • Inability for fight infections due to weakened immune system

Diagnosis of hypothyroidism in dogs can be difficult, and this is one of the over diagnosed diseases in small animal practice. The administration of T4 treatment to a dog that does not require replacement therapy may disrupt normal hormone balance. When diagnosing hypothyroidism, other factors affecting thyroid concentrations are usually taken into account. Many medications for example can lower thyroid function and could cause a potential misdiagnosis. Your veterinarian will take many non-thyroid related diseases and causes into account when examining and diagnosing this disease. The combination of the physical examination along with the results of a blood test may be required in order to make a proper diagnosis.


The initial treatment of choice for hypothyroidism in dogs is levothyroxine

The goal of treatment is to normalize circulating levels of the thyroid hormone. Simple, inexpensive and very effective treatments are available. The initial treatment of choice is levothyroxine because it normalizes both T3 and T4. Dogs that are treated properly with the correct dose and correct monitoring will generally have a normal happy life and the life span is also not usually affected. Skin complications usually resolve in 4 to 6 weeks after treatment is initiated.

The best way to prevent misdiagnosis of hypothyroidism is by keeping the dog from becoming obese. Limiting fattening treats and maintaining a dog’s metabolism through daily exercise can minimize the chances of misdiagnosis.

Next week I will discuss hyperthyroidism in cats which is completely different from hypothyroidism in dogs and has completely different methods of treatment. The best way to keep our dogs healthy and safe is by developing a good relationship with the veterinarian and by asking questions when something is not clear. Medication used to treat hypothyroidism in dogs may include relatively high doses of levothryroxine and those doses will generally be extremely toxic and dangerous if ingested by the family cat. Keeping medications separate and understanding these conditions and the reason for the medications could actually save your pets life. If you have any questions about your pets’ medication you may give one of the 1800PetMeds pharmacists a call so they could help you. Under no circumstance is it a good idea to give a medication that you are not understanding and fully clear about.

Related Posts


  1. Can dogs get osteoporosis as a side effect from taking Levothyroxine for Hypothyroidism like humans can?

  2. Dr. Michael Dym, VMD veterinarianFebruary 27, 2014 at 12:21 am

    Not that I am aware of

  3. I have Hashimoto’s Disease and am amazed to discover that dogs can get it as well. Thanks for the info.

  4. I didn’t know any of this, so thank you.

  5. Dr. Michael Dym, VMD veterinarianApril 4, 2014 at 6:00 pm

    You are very welcome. VERY common in canine medicine

  6. Can senior dogs taking Levothyroxine still take Interceptor for HW prevention without adverse interactions , if so how many hours apart? Just checking as I worry about my 14 year old Sheltie.

    Kind regards,
    Linda Pope

Leave a Comment