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Hypothyroidism In Dogs And Cats

If a dog is truly hypothyroid, it will need hormone
replacement therapy the rest of its life.
NOTE:  The administration of thyroid medication over a long period of time will cause thyroid tissue to atrophy to a variable extent. To simply treat for hypothyroidism without thorough testing is not advised.  Learn more about hypothyroidism in the dog on this page and consult your veterinarian if you have any questions.

Kitten and cat health care questions and answers, articles, videos and images

A note regarding shedding:  In many breeds of both short and long hair dogs episodes of rampant shedding may be totally normal.  Your veterinarian can assist in determining if the shedding your dog is doing is "intense but normal" or if the excessive shedding is due to a medical problem.  Many factors including length of daylight, estrus cycle, post pregnancy shedding, and stress can trigger prolific shedding over a short time period.  In normal shedding, even if big "gobs of fur" are easily removed "right after brushing", the shedding may be normal.  Especially if new hairs are visible in places where shedding has occured, the shedding duration will run its course and new fur will replace the old.  The use of thyroid medication to prevent shedding is not a good idea on a number of levels but especially because it won't work! The use of thyroid medication in dogs that have marginal coats can be very beneficial if they are hypothyroid.  The only way to know if thyroid hormone replacement therapy is proper is to test for T3 and T4 levels in the blood.


Canine hypothyroidism seems to be a topic of conversation within all kinds of dog owning groups.  It is no more prevalent in show dogs, agility dogs, hunting dogs or house pets than any other demographic.

Situated on both sides of the first five to eight tracheal rings, the thyroid gland secretes into the blood stream two major types of thyroid hormones. These active hormones are called T3 and T4. Think of the thyroid gland as the body's thermostat . . . if it is set too high, too much T3 and T4 are secreted and the dog's metabolism burns too hotly. The dog may become hyperexciteable and restless, pant and lose weight.  Hyperthyroidism in dogs is rare; in older cats it is fairly common.

If the thermostat is set too low, subnormal amounts of T3 and T4 are secreted into the blood stream and the dog's metabolic fire is too cool. You can see that if a dog is hypothyroid the signs will be lethargy, weight gain, poor skin and coat condition, shivering and poor stamina.  Many hypothyroid dogs become "watchers" rather than "doers"... they have little interest in engaging in common daily activities.

Not every dog will show ALL the signs, of course. Some will display only one or two of these signs and even then, in borderline cases, some dogs show only very subtle changes from normal.  Hypothyroidism is very rare in cats but can occur if treatment for hyperthyroidism to reduce thyroid function isn't monitored closely.

Control of Thyroid Function - The thyroid gland's activity is directed by outside factors such as the pituitary gland located at the base of the brain and the hypothalamus, another area deep in the brain.  As the dog's metabolism changes due to changes in activity, nutrition, environmental temperature, stress, etc., the need for more or less T3 and T4 is detected by the hypothalamus and pituitary gland - they then send signals to the thyroid to "hurry up" or "slow down".   The brain signals to the thyroid gland via production of TSH (Thyroid Stimulating Hormone) in the pituitary gland at the base of the brain.  The thyroid secretes more or less hormones depending on the signals from TSH.

Now, for the abnormal. Only ten percent of hypothyroidism in dogs is due to any interruption of this supply and demand relationship between the brain-pituitary gland and the thyroid gland.  For example, if the pituitary gland fails due to a tumor or other disease of if the hypothalamus is diseased, the thyroid won't "get the message" to "hurry up". So the thyroid gland, even though it can function normally, doesn't receive the order to do so. T3 and T4 production from the thyroid gland slows down and so does the hypothyroid dog!

Ninety percent of canine hypothyroid cases are due to what is called primary hypothyroidism. The thyroid gland itself is not functioning up to par even when told to do so by the pituitary gland and hypothalamus. The thyroid gland is being told to "hurry up" but simply can't respond.

Causes of Primary Hypothyroidism
1. Lymphocytic thyroiditis- Abnormal infiltration of the thyroid gland with white blood cells. This is likely an autoimmune type of disorder.

2. Idiopathic atrophy - This just means that for unknown reasons the gland degenerates and shrinks in size or becomes infiltrated with fat cells.
Secondary Hypothyroidism
Cancer - Occasionally tumors will occur in and damage the thyroid gland.

Breeds Most Likely to Develop Hypothryoidism

Golden Retriever  Miniature Schnauzer Boxer
Irish Setter Doberman Pinscher Chow
 Poodle Dachshund Malamute
View Michigan State University's veterinary diagnostic lab ranking of 140 breeds with autoimmune thyroiditis here.

Hypothyroidism is disease that does not seem to be due to environmental factors, vaccinations, nutrition (unless a diet deficient in iodine is fed) or other management practices.  It is seen equally in males and females.

Hyopthyroidism effects on canine pyoderma
Crusty, circular patches that spread
are due to a bacterial infection.

Blood tests are essential to a correct diagnosis of hypothyroidism in a dog

Blood tests are essential for making an accurate assessment of the patient.


Images of a dog before and after treatment for hypothyroidism

A dog's skin and coat before treatment for hypothyroidism
An overweight and nearly hairless dog
due to hypothyroidism
A dog's skin and coat after treatment
Two months after starting thyroid hormone replacement therapy
Demodex skin mites in a dog creating hair loss

Your veterinarian may do skin scrapings to be certain the hair loss isn't due to skin mites such as these demodex mites seen under the microscope.


Hyopthyroidism effects on a dog's skin and coat
Hypothyroidism has inhibited new hair growth after the dog shed some hair.  These areas of scant hair regrowth have the same pattern on the left and right side of the dog and is called bilateral, symmetric alopecia.
Hyopthyroidism effects on a dog's skin health
Follicular dermatitis with hair loss is more common in dogs with low levels of thyroid hormone.
Most common signs:
With varying degrees of intensity the most common signs
A dog with poor hair regrowth due th hypothyroidism seen are a thinning of the skin, hair loss over the back and sides without new hair production, and weight gain in the presence of normal caloric intake.

Proof of hypothyroidism is gained via blood analysis for the hormones called T3 and T4 and several other forms of thyroid hormones, if needed.  (See this site for full information). Anemia and high blood cholesterol levels are common.  The veterinarian may make a diagnosis of hypothyroidism based upon the patient's history, physical exam, and laboratory data.

Treatment: Correction of hypothyroidism is aimed at increasing the blood levels of T3 (triiodothyronine) and T4 (tetraiodothyronine). An animal origin thyroid product ("natural thyroid") has no advantage over synthetic medications and, in fact, can be unreliable in some cases.  Synthetic T4, called sodium levothyroxine (or just L-thyroxine) is the best choice for nearly all hypothyroid patients. Giving this medication in tablet form allows normal levels of both T3 and T4 in about 90% of cases. The dose is usually given twice a day but the amount used varies with each patient.

If your dog is hypothyroid and is getting sodium levothyroxine, be sure you are giving it twice a day at the proper dose. If the treatment is working, the dog will "perk up" within a week or two, although skin and coat improvement may take six weeks or longer.  Reproductive status may take six to nine months to stabilize. Follow up blood tests are a good idea to establish the correct dose needed.

As with all hormone related disorders, hypothyroidism can have all sorts of clinical signs, some subtle, some obvious. However, it can be a rewarding and inexpensive disorder to treat. Set that "thermostat" right and the patient often returns to an active... and interactive... happy, healthy dog!

Note:  Many dog owners have had a dog "blow their coat". This excessive shedding over a short period of time is not due to hypothyroidism;  using hormone replacement to prevent this natural occurrence is unfair to the dog.

  arrow  Weight gain
  arrow  Lethargy
  arrow  Cold intolerant
  arrow  Exercise Intolerance
  arrow  Becoming a "watcher" rather than a "doer"
  arrow  Sparse coat over dorsum on both sides
  arrow  Dry, scaly skin
  arrow  Course, dull coat
  arrow  Bilaterally symmetric alopecia
  arrow  "Rat-Tail"   (sparse, bristly hair on tail)
  arrow  "Puppy coat" in adult dog
  arrow  Hyperpigmentation (dark pigment infiltrating the skin)
  arrow  Seborrhea dermatitis (oily, greasy skin)
  arrow  Thin skin when pinched
  arrow  Pyoderma (skin infections
  arrow  Female infertility
  arrow  Male lack of libido
  arrow  Failure to cycle
  arrow  Prolonged interestrus interval
  arrow  Weak, dying or stillborn puppies
  arrow  Silent heat cycles
  arrow  Prolonged estrual bleeding
  arrow  Testicular Atrophy
  arrow  Hypospermia

  arrow  Bradycardia (slowing heart rate)
  arrow  Cardiac arrythmias
  arrow  Cardiomyopathy
  arrow  Corneal lipid deposits
  arrow  Uveitis
  arrow  Keratoconjunctivitis sicca  ("dry eye")

These alopecic areas are due to skin fungal infection

These focal areas of hair loss are not typical of hypothyroidism and are due to a fungal (dermatophyte) infection.

Helpful links regarding hypothyroidism in the dog
  Read about tests done to evaluate thyroid function (.pdf)
  Hypothyroidism in the dog:  Merck Veterinary Manual
  Hyperthyroidism in the dog:  Merck Veterinary Manual
  A veterinary specialist tells us about hypothyroidism

  Learn about cats and hyperthyroidism
Autoimmune thyroiditis has been recognized in dogs and is characterized by destruction of the thyroid gland by an autoimmune process that has both humoral (Type II) and cell-mediated (Type IV) components. The disease is particularly prevalent in Doberman Pinschers, Beagles, Golden Retrievers, and Akitas. Hypothyroidism (Hypothyroidism) may be the sole manifestation of the disease or may be a clinical or subclinical component of a broader autoimmune disorder such as systemic lupus erythematosus, idiopathic polyarthritis, immune-mediated meningitis (periarteritis nodosa), panendocrinopathy, and rheumatoid arthritis.
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