There are a great many factors affecting thyroid hormone levels in dogs. It is essential that a thorough clinical examination is performed and history taken prior to any diagnostic testing. The following factors are also very important to consider when carrying out thyroid function tests.

• Concurrent Therapy – Certain therapies can affect thyroid hormone results and complicate their interpretation. Glucocorticoid and barbiturate medications often cause low total T4 (TT4) concentrations. Where possible, it helps to discontinue these therapies for 1 month prior to thyroid diagnostic testing. When clinical circumstances prevent withdrawal, a diagnostic panel including FT4ED (see below) should be selected. Sulphonamide products can cause a reversible hypothyroidism during their use and, consequently, thyroid diagnostic testing should be postponed until 3 weeks after they have been discontinued.
• Breed Specific Ranges – Sight hounds are known to have lower TT4 levels compared to other breeds. The diagnosis of hypothyroidism in these breeds should be made cautiously and more reliance should be placed on other measures of thyroid function that are less influenced by breed such as TSH (or total T3).
• Low T4 State of Medical Illness (Sick Euthyroid Syndrome: SES) – Dogs with non-thyroidal illnesses will often have low serum TT4 as part of their physiological response to that illness. This is not a T4 deficient state and thyroid supplementation is not appropriate. Instead, it is a reflection of the mechanism used to control metabolic rate during illness that is believed to improve the chances of survival. The effect of non-thyroidal illness on FT4ED values is less common and less dramatic. Ill dogs will often have depressed TT4 levels so the use of additional or alternative thyroid function tests along with careful evaluation of clinical signs and assessment of the likelihood of non-thyroidal illness are important in distinguishing the true hypothyroid dogs from those that are just responding to a non-thyroidal illness.

We recommend that the investigation of possible thyroid disease or dysfunction be performed using panels of several tests rather than individual tests. The diagnostic power of a group of thyroid tests is much greater than that of any available single test. An alternative to the panel approach is the dynamic test approach. Our panels are made up of selections of the following tests: total T4, free T4 by equilibrium dialysis, thyroid stimulating hormone, thyroglobulin antibody and thyroid hormone antibodies.

Individual tests

Total T4 (TT4) – Most dogs with hypothyroidism would be expected to have a low TT4 (~90% of them) making this a test with high, but slightly less than perfect, diagnostic sensitivity. On the other hand, the effects of non-thyroidal illness on TT4 means that many dogs with normal thyroid function will also have a low TT4 making the test poorly specific. Depending on the type of population sampled, up to 25% of dogs with normal thyroid function will yield a low TT4. This poor diagnostic specificity and less than perfect sensitivity means that TT4 has limited value as a stand-alone test for hypothyroidism. Diagnostic power is improved by combining it with TSH measurement or by performing a dynamic response test.

Total T4 also has a role to play in monitoring thyroid therapy where it can be used alone (if hours post-pill and dose frequency are recorded) or ideally in combination with TSH.

Free T4 by Equilibrium Dialysis (FT4ED) – Almost all (>99.9%) of circulating T4 is bound to carrier proteins leaving only a tiny fraction available to interact with tissues. This free fraction can be measured in an ultra sensitive radioimmunoassay following an equilibrium dialysis step. The analysis of FT4ED is the most accurate way of assessing the physiologically important thyroid status of an animal. Samples are dialysed, separating FT4 from serum proteins and protein bound T4. In most cases, TT4 and FT4ED will be highly correlated. The specific circumstances in which they are not are when we would recommend FT4ED as the thyroid hormone test of choice. It would be an advantage to measure FT4ED instead of, or in addition to, TT4 in the following situations:

• Non-thyroidal illness: one of the contributing mechanisms to the low TT4 we see in non- thyroidal illness is an alteration in thyroid hormone-protein binding. Although TT4 concentrations may be greatly reduced, the lower protein affinity for T4 means a higher fraction is available as free hormone and the FT4ED concentration usually remains within the reference range. This makes FT4ED a good test for distinguishing non- thyroidal illness from true hypothyroidism as the cause of a low TT4.
• Concurrent therapies: part of the effect on certain therapies on TT4 is mediated through thyroid hormone-protein binding meaning that FT4ED is less commonly and less dramatically affected by concurrent therapy. FT4ED is the analysis of choice when glucocorticoid or barbiturate therapies cannot be withdrawn prior to embarking on a thyroid diagnostic investigation.
• T4AA: the presence of T4 cross-reacting antibodies (T4AA) in the patient’s serum will interfere with TT4 measurement causing false high values. The FT4ED procedure is unaffected by these antibodies because they are removed by the dialysis step. For an accurate estimation of thyroid status in a dog with T4AA, FT4ED is required. T4AA are present in the serum of approximately 10% of hypothyroid dogs as part of the thyroid pathologic process.

Canine TSH (cTSH) – We expect serum concentrations of cTSH to be high in animals with primary hypothyroidism because the negative feedback effect of thyroid hormones on pituitary production of TSH is lost. Indeed, this is the case most of the time but the diagnostic sensitivity is less than ideal. About 80-85% of hypothyroid dogs will have the expected high cTSH, unfortunately, leaving a proportion that will not. Conversely, cTSH measurement has good diagnostic specificity (up to 100%) meaning that false positives are rare. The combination of thyroid hormone analysis with cTSH measurement makes the most of the advantages of the individual tests while minimizing their deficiencies (see flow chart for interpretation of TT4 & cTSH). Canine TSH may be measured at 30 minutes as part of a TRH stimulation test in the diagnosis of secondary hypothyroidism. Normal dogs should increase cTSH by at least 0.4 ng/mL.

Total T3 (TT3) – The analysis of TT3 is of little value in the diagnosis of hypothyroidism principally because of the high prevalence of cross-reacting T3 autoantibodies (T3AA) in hypothyroid dogs. These antibodies cause false results to be generated in T3 assays.

Thyroglobulin Autoantibody (TgAA) – The presence of TgAA in serum is strongly suggestive of immune mediated lymphocytic thyroiditis which is responsible for more than half of the cases of canine hypothyroidism. In the remainder there is no serological or histological evidence of inflammation. This test is used to document the presence and type of thyroid pathology. It does not provide information on the functional status of the thyroid glands. Evidence of thyroid function does not occur until lymphocytic thyroiditis has destroyed more than 50-60% of thyroid functional mass. Therefore TgAA evidence for thyroid pathology can be seen in animals before dysfunction occurs and while serum TT4 and cTSH concentrations are still normal. Some breeds of dog have a much higher prevalence of serum TgAA indicating a genetic predisposition to immune mediated thyroiditis. The screening of breeding lines or families for TgAA can be helpful for people wishing to breed away from a thyroiditis and hypothyroidism pre-disposition.

Anti T4/T3 Antibodies – Subsets of thyroglobulin antibody exist in certain hypothyroid dogs which cross-react with T4 or T3 assays. These antibodies, lead to falsely high thyroid hormone levels due to interference in the respective assay systems. FT4ED is not subject to this interference.

Thyroid Panels and Dynamic Tests

Basic Thyroid Profile (TP1) – Canine TSH is measured in combination with TT4 and this increases diagnostic accuracy over TT4 alone. This profile is the recommended single sample diagnostic screen.

Bronze Thyroid Profile (TP2) – Canine TSH is measured in addition to a TRH stimulation test. Dynamic function tests such as this have the potential to provide more information than single resting samples.

Silver Thyroid Profile (TP3) – Canine TSH is measured with Free T4 ED. This is a very useful combination for single sample analysis. This offers the advantages of TP4 but without the information on thyroid pathology provided by TgAA.

Gold Thyroid Profile (TP4) – Canine TSH and Free T4 ED are measured in addition to TgAA. This panel does not suffer limitations of interference from non-thyroidal illness, concurrent therapy and T4AA that affect the measurement of TT4. Refer to Platinium profile (TP5) for suggested complete thyroid panel including TT4.

Platinium Thyroid Profile (TP5) – TT4, Canine TSH, Free T4 ED and TgAA. This panel provides the most comprehensive information available from a single sample. The TgAA identifies thyroid pathology and the TT4, FT4ED and TSH provide a complete picture of thyroid function.

Copper Thyroid Profile (TP6) – TT4, Canine TSH and Free T4 ED. This panel provides the most comprehensive information available from a single sample at a more competitive price as TGAA is not measured and the possible cause of the thyroid pathology may not be considered important.

Greyhound Thyroid Panels

Greyhounds and other sighthounds (e.g. Saluki’s) have total serum thyroxine (TT4) levels that are lower than those of other breeds of dog. Thyroid investigation is not uncommon in greyhounds as part of the work up for poor performance, behavioural change, bald thighs and fertility concerns. NationWide Specialist Laboratory have put together profiles of thyroid tests specifically for the greyhound breed.

• Greyhound Thyroid Investigation (GTH1) – Sensitive TT4, TT3, TSH and TGAA
• Greyhound Thyroid Monitoring (GTH2) – Sensitive TT4, TT3 and Canine TSH.

Monitoring Therapy

Thyforon® (Dechra Veterinary Products), Soloxine® (Virbac), Leventa® (MSD) are the only licensed veterinary preparations of levothyroxine. The initial recommended dose varies by datasheet but is around 20ug/kg daily. However, a large proportion of hypothyroid dogs can be successfully treated with a lower dose. TT4 levels should be monitored after several weeks of therapy to determine whether a dose change would be appropriate. For animals receiving therapy twice daily, the time post pill at which the monitoring sample is taken is less important than that it is recorded.

TT4 levels should ideally be between 30-47 nmol/L at around the time of expected peak concentrations (3 hours post pill) and be above 19.0 nmol/L by the time the next pill is due (trough concentration). The half-life of TT4 in the dog can vary but is in the region of 10 to 14 hours.
The measurement of both TT4 and cTSH is recommended in monitoring thyroid therapy in dogs. cTSH reflects the adequacy of thyroid replacement therapy in the preceding days, not just the day of the test. This can help identify inconsistencies in dosing and also prevent inappropriate management decisions being made based on unrepresentative single day TT4 results.

Discounted TT4 estimations are available for practices using Thyforon®. Ask for basic Thyforon® monitor (TT4) or Thyforon® monitor plus (TT4 and TSH).