Serum is the sample required for all canine thyroid hormone tests at the MSU VDL. Two profile bundles are available, canine thyroid diagnostic profile [20010 (standard) or 20011 (premium)] and canine thyroid therapeutic monitoring profile [20012 (standard) or 20013 (premium)]. Premium profiles include free T4 by equilibrium dialysis (fT4D), whereas standard profiles include free T4 by direct serum (fT4). The canine thyroid diagnostic profiles include total T4 (TT4), total T3 (TT3), free T4 (fT4D for premium), thyroid stimulating hormone (TSH), thyroglobulin autoantibody (TgAA), and T3/T4 autoantibody index (T3AA/T4AA). The canine thyroid therapeutic monitoring profile includes TT4, TT3, fT4 or fT4D, and TSH. Autoantibody testing is not included in the thyroid therapeutic monitoring profile.

Fasting, if medically appropriate for the patient, is recommended to avoid lipemia. If lipemia remains despite fasting, submit the sample, and indicate on the submittal form that fasting was attempted. Assays for thyroid hormones are relatively tolerant of lipemia. A moderate degree of lipemia would not be anticipated to affect all results on a profile; therefore, the diagnostic message of the overall results is not likely to be altered.

Free T4 is measured by equilibrium dialysis in premium profiles and includes a dialysis step which filters out interfering proteins followed by an ultrasensitive radioimmunoassay (RIA). Premium profiles are recommended particularly in cases where 1) non-thyroidal illness may be present, 2) the dog receives medication known to potentially affect thyroid hormone concentrations (e.g., glucocorticoids, phenobarbital), 3) the patient is suspected to have autoantibody interference because of a suspicious high T4 result on a screening test, or 4) the patient is known to have T4 autoantibodies by previous testing. Please see the recommendations in the test catalog for sample volume as well as special handling instructions for preparation and shipment of samples for premium profiles.

Canine thyroid diagnostic profiles provide results for a comprehensive evaluation of thyroid hormone concentrations and investigation for the presence of lymphocytic thyroiditis. A diagnosis of primary hypothyroidism is supported by finding thyroid hormone concentrations below respective reference intervals and thyroid stimulating hormone above reference interval. However, a small proportion of truly hypothyroid dogs have low thyroid hormones without a concurrent increase in thyroid stimulating hormone. In these cases, absence of non-thyroidal illness and presence of a strong clinical presentation consistent with hypothyroidism may justify a trial of thyroid replacement therapy. If the clinical picture does not improve despite adequate therapeutic concentrations, thyroid supplementation can be discontinued, and the diagnosis reconsidered.

Borderline low concentrations of only one thyroid hormone concentration, or a standalone increased thyroid stimulating hormone does not provide sufficient evidence for a confident diagnosis of hypothyroidism. Repeat assessment in three (3) to six (6) months is a reasonable approach if clinical suspicion for hypothyroidism is maintained.

Non-thyroidal illness and/or certain medications can cause decreases of thyroid hormone concentrations in euthyroid animals. The most common medications associated with changes in thyroid hormone concentrations in canines are glucocorticoids and phenobarbital. In these cases, thyroid stimulating hormone concentration usually remains within the reference interval. Free T4 as measured by equilibrium dialysis (fT4D) generally exhibits less effect from non-thyroidal illness and/or medication, and premium profiles are recommended in cases with potential confounding factors. Sulfa-containing medications affect the thyroid gland directly and can cause a true but reversible primary hypothyroidism (low thyroid hormone concentrations with concurrent increase in TSH). The pituitary-thyroid-axis will usually recover within three (3) weeks following withdrawal of sulfonamide therapy.

Lymphocytic thyroiditis is an immune mediated disorder believed to be a heritable trait. Characterized histologically by cellular immune infiltration into the thyroid gland, lymphocytic thyroiditis is the most recognized cause of primary hypothyroidism identified in canines. Testing for thyroid autoantibodies is included in our canine thyroid diagnostic profiles.

In samples where the initial thyroglobulin autoantibody result is increased, an additional antibody assay is performed to define the contribution of non-specific immunoglobulin binding. A positive specific binding thyroglobulin autoantibody result is a marker for lymphocytic thyroiditis. Antibodies to T3 and T4 are subsets of thyroglobulin autoantibody which are present in a small proportion of animals with lymphocytic thyroiditis, and these antibodies can interfere with assays for T3, T4 and fT4. Depending on the magnitude of T3 and T4 autoantibody response and assay used, there can be a false increase in measured thyroid hormones. The free T4 by equilibrium dialysis assay is not affected by the presence of T4 autoantibodies and is required to have an accurate measurement of free T4 in animals with positive T4 autoantibody response.

Patients with increased thyroglobulin autoantibody results but thyroid hormone concentrations within reference intervals should continue to be monitored as these dogs are predisposed to hypothyroidism in the future. An inconclusive thyroglobulin autoantibody result does not eliminate the possibility of lymphocytic thyroiditis in the thyroid glands of the patient. A pattern of negative thyroglobulin autoantibody with modestly positive T3 autoantibody response can occur due to poor sample quality.

Healthy sighthounds (e.g., Greyhounds, Salukis, Afghan hounds, Whippets, Borzois, Irish Wolfhounds, Pharoah hounds, Ibizan hounds) may have total T4 and free T4 concentrations which extend below reference intervals established utilizing a general population of canines. Fortunately, the reference interval for thyroid stimulating hormone appears to apply to these breeds. Thus, finding an increased thyroid stimulating hormone concentration with concurrent low thyroid hormone concentrations is required to confidently diagnose primary hypothyroidism in sighthounds. A pattern of low thyroid hormone concentrations and a thyroid stimulating hormone result within reference interval is more likely normal for a sighthound rather than hypothyroidism.

Increased total thyroid hormone concentrations have been documented in normal bitches during periods of progesterone influence (metestrus and pregnancy). Whether these numeric differences are of sufficient magnitude to change the diagnostic message of the entire profile is unknown, but a misdiagnosis of primary hypothyroidism is unlikely.

Therapeutic monitoring profiles provide evaluation of thyroid hormone concentrations but do not include assessment for the presence of thyroid autoantibodies. For dogs known to have T4 autoantibodies, a premium profile will provide accurate assessment for free T4, as the included free T4 by equilibrium dialysis assay is not affected by the presence of T4 autoantibodies.

Steady state levels of thyroid hormones will be reached by two (2) weeks of replacement therapy; however, many clinicians initially assess therapeutic concentrations four (4) to six (6) weeks after beginning thyroid supplementation so that there has been time for clinical responses to also be assessed. It is important to evaluate clinical signs of hypothyroidism for improvement in addition to assessing therapeutic thyroid hormone concentrations.

The interpretation of therapeutic thyroid concentrations is influenced by the timing post-pill at which the sample is obtained. In a dog receiving appropriate thyroxine supplementation, we anticipate peak thyroid hormone concentration (around 3 – 4 hours post-pill) to be towards the top of the T4 reference interval or a little above, and close to or within the lower half of the reference interval at the end of the inter-pill period (8 hours and beyond for dogs receiving twice daily medication or 16 hours and beyond for dogs receiving once daily supplementation). When thyroid stimulating hormone is suppressed into the reference interval, it suggests that overall, therapy must be at least adequate.

Low therapeutic thyroid hormone concentrations with a well suppressed thyroid stimulating hormone result can occur when supplementation is either missed or not well absorbed on the day of the test. The opposite pattern of adequate therapeutic thyroid hormone concentrations with an increased thyroid stimulating hormone result can be seen in cases with recent supplement administration inconsistency, but good compliance on the day of the test.

In dogs with known T3 autoantibodies and/or T4 autoantibodies, it is important to keep in mind the possibility of interference in therapeutic monitoring results. For example, total T3 results may be above reference interval in therapeutic monitoring profiles in the presence of persist significant T3 autoantibodies. TT4 and standard profile free T4 measurements may be falsely elevated with persistence of T4 autoantibodies.

Once an animal is receiving thyroxine, thyroid testing can only tell if the supplement is absorbed. No assessment can be made about the patient’s endogenous thyroid function while being supplemented. Because of the suppressive effects of thyroid supplementation on endogenous thyroid hormone production in healthy dogs, a withdrawal period of up to eight (8) weeks with no thyroid supplement may be needed to confidently assess thyroid function. Animals do not need to be “weaned off” of thyroid supplementation as there is no apparent harm in abruptly discontinuing administration.

The presence of T4 autoantibodies can interfere with assays for total T4 and free T4, causing falsely increased results. The canine thyroid diagnostic profile will provide assessment for autoantibody interference along with concentrations of circulating thyroid hormones and thyroid stimulating hormone. The premium thyroid diagnostic profile includes free T4 by equilibrium dialysis which provides an accurate concentration of free T4 even if T4 autoantibodies are present.

A functional thyroid carcinoma in an aged dog is the most common cause of naturally occurring hyperthyroidism, though this condition in dogs is rare. Most canine thyroid tumors are non-functional in terms of producing excessive thyroid hormones. A diagnosis of primary hyperthyroidism is supported by finding all circulating thyroid hormone concentrations above respective reference intervals, and a thyroid stimulating hormone concentration suppressed to near nondetectable.

Iatrogenic hyperthyroidism can occur with over supplementation of thyroxine, accidental exposure of thyroid supplement dispensed for another pet in the household, or from ingestion of thyroid compounds in the diet. If an external source is suspected, discontinue exposure for a few days and monitor for a rapid decline in thyroid hormone concentrations.