Hypothyroidism In Dogs – Causes, Diagnosis & Treatment

What are the major causes of hypothyroidism?

Sleeping dog on couch

The thyroid gland is located in the front of the throat. It has a number of functions including production of thyroid hormones.

Though there are many reasons why thyroid hormone levels can be low, it is thought that more than 95% of the cases of hypothyroidism are due to destruction of the thyroid gland.


Autoimmune thyroiditis

In dogs, the most common cause of destruction of the thyroid gland is an autoimmune response. For unknown reasons, a dog’s own immune system starts to attack the thyroid gland. This leads to destruction and poor functioning of the thyroid.

Autoimmune thyroididtis is thought to be an inherited disease.


Cancer can cause both hypothyroidism and hyperthyroidism. In the case of hypothyroidism, the cancer can replace the normal thyroid tissue, reducing the number of cells that produce thyroid hormone.

Idiopathic thyroid gland atrophy

This is not very common and is usually either shrinkage of the thyroid gland or deposition of fat in the gland. The cause is unknown.

What other diseases can cause hypothyroidism?

A tumor in the pituitary gland is the most common cause of secondary hypothyroidism. (See TSH level for infomation about the pituitary gland.)

Hypothyroidism can occur in some dogs with cardiovascular problems, including dilated cardiomyopathy, strokes and Von Willebrand disease (blood does not clot properly). Some dogs on steroids (hydrocortisone etc.) can have low thyroid hormone levels.

Diagnosis of hypothyroidism

Any of the symptoms of hypothyroidism, such as poor coat and symmetrical hair loss, lethargy or high cholesterol indicate that a dog could be hypothyroid, and should be tested.

There are two forms of thyroid hormone. The majority is thyroxine (or T4). There is a smaller amount of triiodothyronine (or T3). T4 (which is considered a prohormone) is converted to T3, and is more active than T4.

Your vet may do the Total (baseline) T4 test first, to determine if T4 is low. If it is low, they will do one or more of the other tests to confirm hypothyroidism, and perhaps try to determine the cause. Alternatively they will do a thyroid panel which is usually total T4, free T4 and T3 and/or TSH level.

Note: A low reading on the Total T4 test, by itself, is not sufficient to confirm a diagnosis of hypothyroidism. Low T4 can have a number of causes. Hypothyroidism needs to be confirmed with other tests. In the past hypothyroidism has been over-diagnosed due to basing diagnosis on just the total T4 test.

Total (baseline) T4 test

If there is a problem with thyroid function, then the level of T4 will be lower.

There can be other reasons for T4 being lower, so low T4 by itself is not a diagnosis of hypothyroidism.

(Note: This test is used for monitoring T4 levels in dogs being treated for hypothyroidism)

T4 equilibrium dialysis

This is a more accurate test for T4. Most of the T4 in the blood is bound to protein in the blood. Only the T4 that is unbound (free) is active as a hormone. T4 equilibrium dialysis is a sensitive test that measures the amount of unbound T4, giving a more accurate estimate of the active levels of T4.

TSH level

The levels of thyroid hormones are controlled by TSH (thyroid stimulating hormone). TSH is produced by the anterior pituitary (part of the pituitary gland, which is at the base of the brain). When T4 or T3 are low, more TSH is produced. TSH stimulates production of more of both thyroid hormones.

Since hypothyroidism results in low T4, the anterior pituitary will continually respond to low T4 by releasing TSH. The result is that there will be high levels of TSH in the blood.

For this test TSH in the blood will be measured, to see if it is high.

TSH stimulation test

Another test for hypothyroidism is to inject TSH and see if there are changes in T4.

In a dog with a normally functioning thyroid an increased level of TSH signals the thyroid to release more T4. The result will be that the level of T4 will increase significantly after the injection (usually measured 6 hours after the injection).

In a dog with poorly functioning thyroid, there will be little or no increase in T4, since the thyroid is already producing as much as it can.

Total (baseline) T3 test

If the level of T4 produced is not very low (mild hypothyroidism), the level of T3 may be normal. Since increased levels of TSH (due to low T4) will be stimulating the conversion of T4 to T3, much of the T4 will be converted to T3, so T3 levels may be normal.

A few dogs have problems converting T4 to T3, or have antibodies that destroy T3. This test can be used to determine if there is insufficient T3.

TRH stimulation test

TRH (thyrotropin releasing hormone) is a hormone made in the brain by the hypothalamus, which reacts on the anterior pituitary stimulating it to make TSH.

Like the TSH stimulation test injection of TRH will increase the level of T4 in dogs with normal thyroid function, but not with a poorly functioning thyroid gland. However, the changes in T4 levels in normal dogs are not as great as for the TSH stimulation test, making this test harder to interpret.

Additionally, TRH can have some side effects in some dogs including panting, rapid heartbeat, vomiting and salivation.

Thyroglobulin autoantibodies

Thyroglobulin is a major protein in the thyroid. More than 50% of dogs who have autoimmune thyroiditis will have antibodies against thyroglobulin. However, some dogs that do not have hypothyroidism may have thyroglobulin autoantibodies.

This test can only be analyzed in specialized labs.

Thyroid biopsy

A biopsy of the thyroid is not usually done unless cancer is suspected. Analysis of the thyroid tissue sample can often give clues as to the mechanism of the improper functioning of the thyroid gland.

A thyroid biopsy is a surgery and requires that a dog be put under anesthesia.

Treatment of hypothyroidism

Levothyroxine tablets

Regardless of the cause of hypothyroidism a dog will be given synthetic T4, levothyroxine. Hypothyroidism does not reverse, so once diagnosed a dog will have to take thyroid hormone every day for the rest of their lives.

Depending on many factors such as how low the dog’s own levels of T4 are, and the cause of the hypothyroidism, the amount and the frequency of the doses will vary. Generally the initial dosage is determined by the dog’s weight.

The dog will be monitored by total T4, and physical examination, and the dose adjusted to a dosage optimal for the dog, in order to maintain the level of T4 within the normal range.

T4 is usually measured 4 to 6 hours after giving a regular dose of levothyroxine. This is when there is the highest concentration (peak) of T4 in the blood. On the day of the test (only) the levothyroxine should not be given with food. The reason that no food should be given is that certain foods can slow the absorption of the T4. If the absorption of the T4 is slowed, then testing at 4 to 6 hours will underestimate the peak level of T4.

Since the underlying cause of the hypothyroidism is usually not treated, for many dogs the dose of levothyroxine may need to be increased periodically. After the initial period of adjustment of the dose, most vets will test the T4 levels every 6 months or so.

T3 supplementation

Occasionally, T3 supplementation is required if a dog cannot convert T4 to T3.

In conclusion

Hypothyroidism is a common problem in dogs. Untreated, it will interfere with the quality of life of a dog, and can cause other health problems. Hypothyroidism used to be over-diagnosed. Treatment involves taking levothyroxine for life.

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