Endocrinology
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Lymphocytic Thyroiditis (Autoimmune Thyroiditis)
Lymphocytic thyroiditis is the underlying cause in many cases of primary hypothyroidism in dogs and the predisposition to its development is believed to be highly heritable. It is an immune mediated disorder characterized histologically by a diffuse infiltration of lymphocytes, plasma cells, and macrophages in the thyroid gland. Antibodies interact with the follicular cell, colloid, or thyroglobulin antigens and activate the complement cascade and cell-mediated cytotoxicity. The progressive destruction of follicles and secondary fibrosis eventually leads to a failure of thyroid hormone production. More than 60 or 70% of the thyroid tissue needs to be destroyed before we see changes in laboratory measures of thyroid function. This process can take months or years to cause classic hypothyroidism. In some animals it may not progress. Anti-thyroglobulin antibodies are released into the circulation in animals with this condition. A test for these antibodies in serum is included in the MSU VDL Canine Thyroid Diagnostic profile. This laboratory is not aware of any evidence to support the hypothesis that vaccination could be associated with the induction of immune mediated thyroid disease. In fact, some researchers have looked for and failed to find such an association (Hogenesch et al., 1999 Advances in Veterinary Medicine 41: 733-747).
Interpreting MSU’s Diagnostic Thyroid Profile Results
In addition to testing for serum levels of total thyroxine (TT4), total tri-iodothyronine (TT3), and free thyroxine (FT4), this profile tests for T4 antibodies (T4AA), T3 antibodies (T3AA), canine thyrotropin (cTSH; thyroid stimulating hormone), and thyroglobulin antibodies (TgAA). The cTSH test provides much needed information in any attempt to diagnose hypothyroidism. Many non-thyroidal factors can cause decreases of TT4, TT3, and FT4 into the hypothyroid range in a dog with normal thyroid function making it difficult to differentiate sick-but-euthyroid animals from those with hypothyroidism. When thyroid hormone levels are low due to primary hypothyroidism, most (around 75%) animals will have abnormally high cTSH levels. The antibodies (T3AA, T4AA, and TgAA) are markers for lymphocytic inflammation within the thyroid gland. T3AA and T4AA are subsets of TgAA which are present in only a proportion of TgAA positive animals. The T3AA and T4AA cross-react with T3 or T4 in immunoassays and cause false results in some thyroid hormone assays. It is, therefore, crucial to know whether they are present before interpreting thyroid hormone results. A step-by-step approach to interpreting the 7-parameter profile would include:
Normal Thyroid Hormone Concentrations with an Elevated Thyroglobulin Autoantibody Result
This most likely reflects the presence of subclinical thyroiditis (but very rarely might indicate thyroid neoplasia in an older dog).
In these cases, there is evidence of pathology within the thyroid glands, but not to the extent that it is interfering with thyroid function. Histologically, there are often small foci of lymphocytic inflammation scattered through otherwise healthy appearing thyroid tissue. We estimate that it takes >60% destruction of thyroid tissues by the inflammatory process before we see changes in laboratory measures of thyroid function. It appears that the progression to overt hypothyroidism in these cases is often very slow and that, indeed, some may never have progression of pathology but remain euthyroid for many years.
Measuring T3 and T4 Autoantibodies
In dogs, antibodies which cross-react with T4 and/or T3 are markers for lymphocytic thyroiditis. These antibodies are generated against T4 and T3 containing epitopes on the thyroglobulin molecule, i.e., they are subsets of TgAA. Positive values indicate thyroid gland pathology, and also tell us about the validity of the thyroid hormone results. T3AA and T4AA are present in around 35% and 14% of hypothyroid dogs respectively. In the MSU VDL Endocrinology Laboratory, T3 antibodies cause a false decrease in TT3. T4 antibodies falsely increase TT4 and may increase FT4 results in the standard thyroid profiles. In a hypothyroid dog with T4AA, T4 may be falsely elevated into the normal range and the true diagnosis masked. To obtain an accurate measure of T4 status in a T4AA positive animal, submit a sample for free T4 by equilibrium dialysis (FT4d) which is not affected by antibody. FT4d is included in Premium Thyroid Profiles. Although these antibodies have a large effect in the laboratory test tube, the clinical impression is that they do not interfere significantly with the availability of thyroid hormone in a thyroxine-treated hypothyroid dog.
Premium versus Standard Profile
The Premium profile is a 7-parameter profile that differs from the Standard 7-parameter profile only in that it measures free T4 by an equilibrium dialysis method rather than by a two-step radioimmunoassay employed in the Standard Canine Thyroid Diagnostic Profile. This technique is elaborate and time consuming, but allows a more specific measurement of FT4 that is not affected by antibody interference, and is less commonly affected by certain medications or non-thyroidal illness than total T4 or standard measures of free T4. Choose this test if: 1) T4AA have been documented, 2) non-thyroidal illness is known to be present, or 3) the dog has received interfering substances such as steroids or phenobarbital. Premium and Standard versions are also available for our Feline, Other Species, and Canine Thyroid Monitoring Profiles.
Free T4 by Dialysis
The FT4 by dialysis test is indicated to better identify animals that are euthyroid but whose total- or standard free- T4 is falsely increased due to T4 antibodies or physiologically decreased due to general/systemic illness. A dialysis step first filters out the large antibodies and hormone binding proteins to provide a measurement technique unaffected by antibodies or by changes in T4-protein binding which can occur during non-thyroidal illness. FT4d is useful in cases were total- or standard free- T4 is borderline low and TSH is normal to assist in the differentiation of euthyroid but sick dogs from those 20-25% of hypothyroid dogs which could have a normal TSH result. FT4 by dialysis may rarely be increased in systemic illness due to decreased protein binding affinity for T4. FT4d is less commonly suppressed by non-thyroidal illness, but does occasionally fall below the reference range in some sick but euthyroid animals. An important point to recognize is that results from both types of free T4 assays are highly correlated. Most samples that are normal in one assay will be normal in the other. Free T4 will be low with both assay methods in most hypothyroid dogs. There may be a slight improvement in diagnostic sensitivity for feline hyperthyroidism with free T4 by equilibrium dialysis, especially when seen in conjunction with a total T4 assay result. Situations in which measurement of free T4 by dialysis are likely to have an advantage over standard measures include:
Measurement of Thyroid Stimulating Hormone (TSH)
Thyroid stimulating hormone (TSH) is released by the pituitary in response to low serum T4 levels, due to loss of negative feedback. An increase in TSH concentration therefore indicates a failure of adequate thyroid hormone production (hypothyroidism). Abnormally high concentrations of T4, as seen in hyperthyroidism (a very rare condition in the dog) will inhibit TSH secretion and cause low serum concentrations of TSH. Unfortunately, current assays for canine TSH do not have sufficient sensitivity to differentiate normal from low concentrations. It is for this reason that the low end of the canine reference range extends to zero and the test cannot be used reliably to identify hyperthyroid states.
Low Thyroid Hormone Concentrations with Non-Elevated Thyroid Stimulating Hormone
Depending on the clinical presentation, one of two main possibilities is likely. With this pattern of test results, a trial with T4 supplementation is suggested only if there is a strong clinical presentation consistent with hypothyroidism and if no non-thyroidal illness can be detected. An objective case review should be conducted after 6-8 weeks of therapy for evidence of clinical improvement. Thyroid supplementation can be discontinued if no improvement has occurred in that time, and the diagnosis reconsidered. A therapeutic monitoring sample taken during treatment should help confirm whether adequate amounts of thyroid medication were being absorbed. Even when there is clinical improvement, strictly speaking, therapy should be discontinued to see if the original clinical signs return in the absence of medication. Admittedly, this latter protocol is rarely followed in clinical practice. While the administration of thyroid hormone to animals which do not have hypothyroidism is generally considered to have minimal risk, large scale studies in human medicine have shown detrimental effects of such treatment in patients that have decreases in serum thyroid hormone concentrations due to non-thyroidal illness. (Brent GA and Hershman JM. Thyroxine therapy in patients with severe non-thyroidal illness and low serum thyroxine concentrations. J Clin Endocrinology and Metabolism. 1986, 63:1)
Normal Thyroid Hormone Concentrations with Elevated Thyroid Stimulating Hormone
We are aware of a few circ*mstances which will cause this pattern of test results. However, there may be other possibilities about which we have yet to learn. Spurious elevations in TSH are occasionally seen as a non-repeatable physiological anomaly. Physiologic explanations for this pattern of test results include recovery from significant non-thyroidal illness when that illness had been associated with low serum thyroid hormone concentrations, and recovery from a reversible sulfa-drug-induced hypothyroidism. In these instances, elevated serum TSH concentrations are observed as the pituitary encourages the suppressed thyroid glands to return to function. A similar finding might follow withdrawal of prolonged thyroid medication from a dog which had normal thyroid function. For this reason, in cases where a dog has been receiving thyroid supplement and the original diagnosis of hypothyroidism is questioned, thyroid supplementation should be stopped for approximately 4-6 weeks (depending on duration and dose) prior to diagnostic testing. The principal pathologic explanation for this combination of test results is the presence of subclinical hypothyroidism (partial thyroid failure). When the functional reserve of the thyroid glands is decreasing (>60% destruction), the first physiologic response is to increase TSH production and encourage the small amount of remaining thyroid tissue to work much harder in an attempt to maintain normal serum thyroid hormone concentrations. Often, this compensatory mechanism is successful and low-normal thyroid hormone concentrations are maintained by the increased TSH production for many months, despite the "partial" thyroid failure. Because normal thyroid hormone levels are being maintained, it is not yet clear whether we should expect to see clinical signs of dysfunction in thyroid hormone dependent tissues. Those clinical signs which are present in these animals may reflect another non-thyroidal illness and consequently a therapeutic trial might not result in marked clinical improvement. The best approach in these cases is to retest at a later date. If subclinical hypothyroidism is present, the elevated TSH is likely to be persistent and T4 concentrations may fall further with time as the decrease in thyroid functional mass continues. If the animal is recovering from a serious illness or the effects of medication, TSH can be expected to return to reference range and T4 concentrations will remain normal.
The Only Abnormality in a Thyroid Profile is a Low Total T3 Concentration
First check T3AA to see if the TT3 result can be trusted to be valid. If T3AA is positive, then the interfering effect of this antibody is the most likely explanation for the low TT3 result. See above for the meaning of positive antibodies and the further interpretation of the 7-parameter profile. If T3AA is negative, consider the following: when comparisons are made between the performance characteristics of T4 and T3 in their ability to differentiate normal from hypothyroid dogs, the T4 assays generally perform better. In the absence of T4 cross-reacting antibodies, T4 results more directly reflect thyroid hormone production by the thyroid glands than T3, whose concentration is heavily based on modulation of peripheral de-iodinases responsible for T4 to T3 conversion. Such a finding may reflect a "low T3 state of medical illness" suggesting a significant non-thyroidal illness. The opposite situation, of low T4 with normal T3 concentrations in the absence of antibodies, also occurs. In this case, the fact that T3 concentrations are being maintained makes it unlikely that the animal would have clinical signs of hypothyroidism. We often attribute a combination of low T4, normal T3 and normal TSH concentrations to a "low T4 state of medical illness". Whether there is any pattern to which dogs or which diseases are associated with Low-T4 or Low-T3 (or both) states of medical illness has not been determined.
Drugs that Affect Thyroid Hormone Concentrations
Phenobarbital and glucocorticoids can cause a "sick euthyroid" pattern of low thyroid hormone levels without an increase in TSH. The mechanism by which phenobarbital exerts its effect are not clear but may include a central component. In the long-term administration of phenobarbital there may even be slight decreases in FT4d and increases in TSH. Glucocorticoids can suppress TT4 and FT4, but their effect on FT4d can be inconsistent and less dramatic suggesting that their principal effect is through alterations in protein binding. Sulfa-containing medications affect the thyroid directly to decrease T4 and T3 production, and cause a consequent increase in TSH. This is a true but reversible hypothyroidism. The pituitary thyroid axis recovers in less than 3 weeks following withdrawal of the medication. Reports of a minimal effect of carprofen (Rimadyl) on TT4 exist. FT4d is less affected. However these studies have not included control animals and the existence of a biologically significant effect remains controversial. Once an animal is receiving thyroid hormone medication, thyroid testing can only tell if that medication appears to be well absorbed. No comment can be made about the patient’s endogenous thyroid functional status. Because of the suppressive effects of thyroid medication on the endogenous thyroid in normal dogs, a withdrawal period of 6-weeks is recommended before a diagnostic sample can be taken with which the dogs' endogenous thyroid status can be assessed free from interference by the thyroid medication.
Effect of Estrus or Pregnancy on Thyroid Profile
Higher total thyroid hormone concentrations have been documented in normal bitches during periods of progesterone influence (metestrus and pregnancy). However, whether these numeric differences are of sufficient magnitude to cause misdiagnoses is yet to be determined but perhaps unlikely. The effect of reproductive stage on TSH has not yet been reported.
Total T4 and/or Free T4 is (are) Low but Everything Else is Normal
We generally attribute this kind of finding to the "low-T4 state of medical illness" suggesting the presence of a significant non-thyroidal illness. This lowered thyroid hormone concentration we believe is due to the effects of the illness on hormone production and/or serum protein binding and perhaps excretion. The opposite situation of normal T4’s but low T3’s may also be observed.
Some Dog Breeds may have Different Thyroid Hormone Reference Ranges
Sighthounds (Greyhounds, Salukis, Afghan hounds, Whippets, Borzois, Irish Wolfhounds, Pharoah hounds, Ibizan hounds, etc.) can have lower TT4 and FT4 reference ranges although these are generally not published. In some case studies, the TT4 reference range extends to zero. As such, a dog with hypothyroidism cannot be differentiated from a normal dog using T4 tests alone. Fortunately, general canine population ranges for TSH appear to usually apply to these breeds. Thus a sighthound with low TT4 and FT4 concentrations, but with a normal TSH concentration, is unlikely to be hypothyroid and does not require thyroid supplementation.
Using Steroids to Treat Lymphocytic Thyroiditis
Treating with corticosteroids might be desirable to decrease the immune/inflammatory components of this disease, but our laboratory does not advocate their use. There are no published studies evaluating this approach in dogs. There are more disadvantages to steroid use than advantages, because of their many side effects.
Thyroglobulin Autoantibody, T3 Autoantibody, and T4 Autoantibody Reported as Percentage
Thyroglobulin autoantibody, T3 autoantibody, and T4 autoantibody are reported as a percentage of the value of the positive control for the assay. For TgAA, samples that are > 35% of the positive control value are considered to be positive for the presence of lymphocytic thyroiditis.
Assessing Nonspecific Binding
In most assays, there is specific binding, and nonspecific binding which is usually low. This nonspecific binding can vary from individual to individual, and may be high enough to cause an apparently positive TgAA value. In cases where the TgAA result is at least 20%, an extra test to evaluate the contribution of nonspecific binding is performed. In many instances, the specific TgAA (sTgAA) is still positive indicating lymphocytic thyroiditis. However, in some instances, the specific TgAA becomes negative when nonspecific binding is taken into account.
Determining Thyroid Status in Dogs Receiving Thyroid Supplement without Hypothyroidism Diagnosis
Once an animal is receiving thyroid hormone medication, thyroid testing can only tell if that medication appears to be well absorbed. No comment can be made about the patient’s endogenous thyroid functional status. Because of the suppressive effects of thyroid medication on endogenous thyroid hormone production in normal dogs, a withdrawal period of 4-6 weeks with no thyroid supplement is recommended before a diagnostic sample can be taken. There is no need to wean. After 4-6 weeks without thyroid supplement, the dogs' endogenous thyroid status can be assessed, and is free from interference by thyroid medication.
Thyroid Mass with Normal to Low Thyroid Hormone Concentrations
Most thyroid masses in the canine are nonfunctional and do not secrete excess concentrations of thyroid hormones. The majority of thyroid tumors are thyroid carcinomas, and only a small percentage result in hyperthyroidism. Occasionally, a thyroid mass may cause an increase in TgAA.
Aggression and Hypothyroidism
There have been anecdotal reports of hypothyroidism resulting in aggression and it may be that the effects of hypothyroidism can make dogs more irritable. However, an association of hypothyroidism with aggression has not been proven in any retrospective or prospective studies where large numbers of dogs have been evaluated. In one retrospective study performed here at Michigan State University, there was actually a negative correlation between a history of aggression and hypothyroidism. In other words, dogs with a history of aggression were actually less likely to be hypothyroid.
