The thyroid gland is found near the trachea (windpipe), just below the larynx (voice box). It is a paired gland that is responsible for the production of thyroid hormones. The major thyroid hormone that is produced by the thyroid gland is thyroxine (T4). A small amount of another thyroid hormone, triiodothyronine (T3), is also made by the thyroid gland. Thyroid hormones have far-reaching effects on the body, in essence governing the body’s metabolic rate.
The function of the thyroid gland is controlled by the pituitary gland located at the base of the brain, through a hormone called TSH (thyroid stimulating hormone).
The most common dysfunction of the thyroid gland in dogs is associated with decreased production of thyroid hormones. This condition is called hypothyroidism.
Hypothyroidism may occur due to two main causes: lymphocytic thyroiditis (an inflammatory condition) or idiopathic thyroid gland atrophy (shrinkage of the gland due to an unknown cause). These two causes account for approximately 95% of the cases of hypothyroidism. With either cause, the thyroid gland eventually is unable to make sufficient thyroid hormone.
The remaining causes of thyroid hormone disorders are rare, and include failure of the pituitary gland to stimulate the thyroid gland to produce adequate hormone (secondary hypothyroidism), as well as congenital (birth) defects in thyroid hormone production, and cancer of the thyroid gland.
Thyroid hormones have widespread effects on the body, and are involved in the metabolism of food as well as the daily metabolic functions of most of the body’s tissues and organs. Therefore, the effects of decreased thyroid hormone concentrations may be seen in many initial blood and serum screening tests. Changes in the initial screening tests help identify the presence of hypothyroidism, and may also uncover other conditions related to or unrelated to decreased thyroid hormone production.
Initial screening tests:
Complete blood count (CBC):
The white blood cells, red blood cells, and platelets (cellular components involved in the clotting process) are evaluated in this test. Occasionally a mild anemia (low red blood cell count) may be seen with hypothyroidism because thyroid hormone is required for optimal red blood cell production. Changes in the appearance of the individual red blood cells (the so-called target cells) may also be noted. Target cells form because of increased cholesterol deposits in the membrane or outer cell layer of affected red blood cells. The increased cholesterol is related to the hypothyroid state (see below). While target cells are not specific for the diagnosis of hypothyroidism, their presence is supportive of the diagnosis.
Hypothyroidism may result in compromised immune function, so occasionally an increased number of white blood cells may be found as a result of some concurrent inflammatory or infectious condition.
Serum biochemistry profile:
Serum (the liquid portion of blood) contains enzymes, proteins, lipids (fats), glucose (sugar), and metabolites. These serum components are derived from different organs such as the liver, kidney, and pancreas, and can give us an indication of the function of these organs.
The main feature noted on the serum biochemistry profile in approximately 75% of hypothyroid dogs is an increase in cholesterol. This hypercholesterolemia occurs because thyroid hormones are required for both the production and breakdown of lipids (fat). Lipid breakdown is decreased in the hypothyroid state, so that fat accumulation may occur in the circulation.
Occasionally mild changes in liver and muscle related enzymes may be noted but these changes are not consistently seen with hypothyroidism.
Evaluation of urine does not usually provide any specific information pertaining to a diagnosis of hypothyroidism. It is important however, to evaluate a urine sample in any dog that is ‘unwell’ so that urinary tract disorders can be eliminated as a cause of the dog’s illness.
If we find changes characteristic of hypothyroidism with these initial screening tests, then more extensive diagnostic tests will be indicated to confirm the diagnosis of hypothyroidism.
Specific tests for hypothyroidism:
The thyroid gland produces both thyroxine (T4) and triiodothyronine (T3) hormones. Thyroxine (T4) is the primary hormone produced by the thyroid gland in response to stimulation by the pituitary gland. Although it is also produced by the thyroid gland, T3 is primarily made in the tissues by the breakdown of the T4 hormone. Therefore, evaluation of T3 levels is not commonly useful in the diagnosis of hypothyroidism.
A ‘feedback system’ exists between the thyroid gland and the pituitary. When T4 concentrations in the blood are low, the pituitary sends a signal to the thyroid gland using thyrotropin, also known as thyroid stimulating hormone (TSH), prompting the thyroid gland to produce more T4. When T4 levels in the circulation are adequate, the pituitary decreases its production of TSH, thereby reducing the production of T4.
Total T4 (Thyroxine): T4 circulates in the blood in two forms; one form of the hormone is bound, or attached to proteins in the blood, while the other form circulates freely within the blood stream. Total T4 measures both forms of the hormone in a blood sample. If the total T4 concentration is well within the normal range, then your dog is not hypothyroid. If the total T4 concentration is at the low end or below the normal range, and your dog has supportive clinical signs, then hypothyroidism is likely. Unfortunately there are conditions other than hypothyroidism that may cause the total T4 value to be reduced. These conditions include the presence of another illness or the administration of some drugs, such as anti-inflammatory medications.
Free T4 (free thyroxine): This test measures the amount of the free thyroxine hormone in a blood sample. The advantage of free T4 is that this measurement is less affected by the presence of other illnesses or drug therapies and therefore offers more reliable information regarding the status of the thyroid gland.
If the free T4 is within the normal range, then your dog does not have hypothyroidism. If the free T4 is below normal range, and your dog has supportive clinical signs, then hypothyroidism is likely.
The disadvantage of this test is that the blood sample must be sent to a referral laboratory in order to perform the free T4 assay using a special technique. Because of this, the free T4 assay is more costly and takes longer to get a result. Therefore, it is common to first evaluate a blood sample for the total T4. If the results of the total T4 are ambiguous, then a free T4 test can be done.
Endogenous TSH (thyrotropin): TSH is produced by the pituitary gland. In hypothyroid dogs, the concentration of TSH may be increased as the pituitary tries to stimulate the thyroid gland to increase hormone production. TSH can be measured in a blood sample but the result needs to be interpreted along with a simultaneously measured total T4 or free T4 result. Measurement of TSH requires sending the blood sample to a referral laboratory.
A normal TSH, free T4, and total T4 effectively rules out the possibility of hypothyroidism.
Although there are other tests available for the assessment of thyroid function, the above tests are the most commonly used for the diagnosis of hypothyroidism and yield the most diagnostic information.
In addition to being used for the diagnosis of hypothyroidism, total T4 can be used to monitor therapeutic levels of thyroid hormone supplementation if your dog requires thyroid hormone replacement therapy.
Additionally, in the very rare cases of suspected thyroid gland cancer, total T4 can also be measured to determine whether or not the cancerous tissue is producing excessive amounts of thyroid hormone.