What initial tests are used to diagnose hypothyroidism in dogs?
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 bodys tissues and organs. Therefore, the effects of decreased thyroid hormone concentrations may be seen in many initial blood and urine screening tests. Changes in the initial screening tests help identify the presence of hypothyroidism, and may uncover other conditions, some of which may be related to decreased thyroid hormone production while others may indicate other, non-related problems.
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. 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 because of some concurrent inflammatory or infectious condition.
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.
Urinalysis. 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 dogs illness.
If your veterinarian finds changes characteristic of hypothyroidism with these initial screening tests, then more extensive diagnostic tests will be indicated to confirm the diagnosis of hypothyroidism.
Step 2: Screen With a T4 Concentration
Total T4 concentration is a useful screening test for hypothyroidism. The sensitivity of this test for the diagnosis of canine hypothyroidism is reported to be 89% to 100%.9-12 If the T4 concentration is well within reference range, it is very likely the dog is euthyroid and further thyroid testing is not required. Free T4 (fT4) and thyroid-stimulating hormone (TSH) are evaluated only if the T4 concentration is low (FIGURE 2). Combined T4, fT4, and TSH testing is not recommended at this stage and may add unnecessary expense since a normal T4 concentration effectively rules out hypothyroidism.
However, a T4 concentration below reference range is not diagnostic for hypothyroidism. In addition to normal daily fluctuations, several medications have been demonstrated to lower the serum T4 concentration of dogs (BOX 2), and some also affect fT4 and TSH concentrations. Certain drugs, such as trimethoprim–sulfamethoxazole, can have direct effects on the pituitary–thyroid axis and result in hypothyroidism.13 Furthermore, nonthyroidal illnesses can alter thyroid hormone metabolism and result in the euthyroid sick syndrome (BOX 3). Concentrations of fT4 are less likely to be affected by concurrent illness, but if the illness is severe enough, fT4 can also be low.21,22 Therefore, thyroid testing should not be performed in dogs that are systemically ill. If a dog with a concurrent illness is tested for hypothyroidism, test results should be interpreted with caution.BOX 2 Drugs That Alter Canine Thyroid Hormone Function or Test Results13-20
BOX 3 Euthyroid Sick SyndromeThis syndrome refers to a condition in which nonthyroidal illness suppresses the concentration of circulating thyroid hormones. The mechanism is complex and likely involves changes in hormone distribution and metabolism and altered binding of hormones to proteins.
Because diagnosing hypothyroidism is not an emergency, sending samples out to a reference laboratory is advisable. Since additional confirmatory tests are required, it is helpful to collect and hold extra serum when collecting for the T4 test.
It is important to remember that “normal” reference ranges for T4 do not apply to sighthounds, as healthy dogs of these breeds have lower T4 concentrations than other breeds.23,24
Dr. Bruyette says, “If you are super-confused by all of this, what test do you want to run? You want to run fT4 by EqD. Thats the one that will give you the right answer most of the time in dogs with nonthyroidal disease and in dogs that are taking medications.”
Free thyroxine is the fraction of T4 unbound to plasma thyroid hormone binding proteins that is proportional to cellular hormone and undergoes conversion to the more biologically active form of thyroid hormone (ie, T3). The pituitary also monitors serum fT4 concentrations as a primary negative feedback signal in determining thyrotropin (TSH) secretion. Results of past studies comparing the accuracy of serum fT4 concentrations versus total T4 concentrations for assessing thyroid gland function have been conflicting, presumably because of differences in the technique used to measure serum fT4 concentration.
The breed of dog can also affect the TT4 normal range. Sight hounds (greyhounds, salukis, basenjis) do not have a lot of TGB in their blood, so their TT4 is about 50% of normal. Therefore, you cannot use a typical reference range to diagnose hypothyroidism in these breeds. And, as mentioned, you should never diagnose hypothyroidism based solely on a low TT4 anyway. Dr. Bruyette says, “Were working to get to breed-related normal ranges. To say that TT4 is 1 to 4 μg/dl for all breeds, across all ages, thats crazy. We need to have labs give you a reference range for the breed of the dog.”
Hypothyroidism is more common in females than in males, which is true of all autoimmune endocrine diseases. Spaying and neutering has no effect on prevalence.
Editors note: Want the latest and greatest in the realm of canine hypothyroidism? Check out the supplimentation and monitoring updates here.