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Canine and feline thyroid disease (Proceedings)
In the last few years it is still apparent that the best test to use in the initial approach to the patient with hyperthyroidism is measurement of total T4 (TT4) concentrations.
A. Elevated T4 concentration. Measurement of T3 of little help.
B. May be able to palpate a thyroid nodule before the T4 is elevated. Recheck T4 3 every 3 – 6 months or when signs occur.
C. Occasionally cats with hyperthyroidism may have a T4 in the normal range at the time of sampling. This is especially true in cats with mild hyperthyroidism. A second sample may be needed in those cats with strong clinical evidence of thyrotoxicosis. The second sample should be taken a few days to weeks later, as more pronounced fluctuations in thyroid hormone levels occur over days rather than hours. Nonthyroidal illness may also result in highnormal serum T4 concentrations even in the face of hyperthyroidism. Following correction of the underlying illness or discontinuation of medications, T4 levels will increase into the hyperthyroid range.
In animals in which hyperthyroidism is suspected, but the basal T4 levels are consistently normal, four additional tests can be considered.
1. T3 Suppression Test:
A. Basis of the Test: The normal pituitarythyroid axis will be suppressed following supplementation with T3. A decrease in TSH concentration will lead to a decrease in T4 levels.
B. Performing the Test
1. Determine basal T4 level.
2. Administer T3 (25 ug) every 8 hours for two days, giving the last dose on the morning of day 3.
3. Determine T3 and T4 concentrations 4 hours following the last dose of T3.
4. Normal cats:
a) T4 levels suppress greater than 50% from pretreatment value.
2. TRH Stimulation Test:
A. Basis of the Test: TRH is the hypothalamic peptide that regulates TSH release from the pituitary. TSH response to TRH is blunted in patients with hyperthyroidism.
B. Performing the Test:
1. Obtain basal T4 level.
2. Administer 0.1 mg/kg TRH IV.
3. Obtain 4 hour post TRH T4.
4. Normal cats:
a) Twofold rise in T4 post TRH
b) Hyperthyroid cats have minimal to no increase in T4.
3. Free T4
A. In cats where the TT4 is in the upper 50% of the basal resting range, an elevated fT4ED in the face of clinical signs is highly predictive of hyperthyroidism. Use of fT4ED should not be used as the initial screening test as some euthyroid senior cats have have elevated fT4ED. Due to the simplicity of the test, fT4ED should be the first line test in diagnosing cats with hormonally occult (normal TT4) hyperthyroidism.
A. Technetium scans may be helpful in hormonally borderline cases where bilateral uptake is clearly increased or unilateral disease is present.
In the last few years it is still apparent that the best test to use in the initial approach to the patient with hyperthyroidism is measurement of total T4 (TT4) concentrations. TT4 testing is simple and inexpensive and will provide the correct diagnosis in the majority of feline patients presented for evaluation. However, we are now faced with attempting to diagnose or confirm hyperthyroidism in cats that are asymptomatic, have only mild clinical signs, and/or have concurrent illness that may affect accurate laboratory assessment of thyroid function. These cases can be very challenging though recent work seems to indicate that measurement of free T4 by equilibrium dialysis (fT4ED) represents the logical next step (though it should be emphasized, not the first step) in the approach to these patients. This approach will likely eliminate the need for additional expensive or problematic tests such as TRH stimulation and T3 suppression testing. We also have seen recent work on the effects of thyrotoxicosis on bone and calcium metabolism and how hyperthyroidism can affect our laboratory assessment of concurrent diseases such as diabetes.
Two excellent papers have assessed the value of fT4ED in the diagnosis of hyperthyroidism in cats and/or the effects of non-thyroidal illness on thyroid function. As is the case in dogs, euthyroid cats with non-thyroidal illness may have a decrease in TT4 levels that is most likely the result of protein binding abnormalities. In a study looking at 98 cats with non-thyroidal illness and 50 normal control pet cats thyroid function was assesses by measurement of TT4 and fT4ED. T4 concentrations were measured by radioimmunoassay, and serum free T4 concentrations were measured by direct equilibrium dialysis. Serum total T4 concentrations were significantly (P < 0.001) lower in sick cats (mean SD, 17.18 8.14 nmol/L), compared with healthy cats (mean SD, 26.00 7.62 nmol/L). Serum total T4 concentrations were inversely correlated with mortality. Differences in serum free T4 concentrations in sick cats (mean SD, 27.70 13.53 pmol/L), compared with healthy cats (mean SD, 24.79 8.33 pmol/L), were not significant. A few sick cats had serum free T4 concentrations greater than the reference range. This study showed that as is the case in dogs and man, euthyroidism is maintained in sick cats, despite low serum total T4 concentrations. In addition, measurement of serum total T4 concentrations was a valuable prognostic indicator as it appeared to be an excellent predictor of mortality. Lastly and perhaps more importantly with respect to diagnosing hyperthyroidism, some euthyroid older cats have elevated fT4ED concentrations. This would indicate that initial use of fT4ED as a screening test for hyperthyroidism in older cats can lead to false positive results.
One of the challenges in diagnosing hyperthyroidism is the effect of non-thyroidal illness on thyroid function tests in cats with concurrent hyperthyroidism. In a recent very large study TT4, ft4ED and T3 concentrations were measured in 917 cats with hyperthyroidism, 221 cats with non-thyroidal illness, and 172 clinically normal cats. Ft4ED was significantly more sensitive (0.985) then TT4 (0.913) as a diagnostic test for hyperthyroidism, however of the 221 cats with non-thyroidal illness, 12 cats had a high fT4ED (false postive). Therefore the calculated specificity of fT4ED as a diagnostic test for hyperthyroidism was significantly lower (0.937) than the specificity of TT4 (1.0). This study indicates that measurement of fT4ED is only indicated in those cats with clinical signs and a TT4 in the upper 50% of the normal resting range. It appears that concurrent non-thyroidal illness in some cats with hyperthyroidism may be sufficient to drop TT4 values into the upper half of the normal resting range. In these animals the fT4ED will be elevated. In our experience we see this most commonly in cats with moderate to severe GI disease (IBD, lymphoma) or in cats on concurrent glucocorticoids. The biggest challenge to the clinician is on deciding on how to treat such cats appropriately. In general, one must decide what role both diseases are playing with respect to the clinical signs and address each disease separately.
We have known for a long time that many cats with hyperthyroidism have elevated ALP and/or ALT concentrations. Bile acids and liver biopsies in these cats are generally clinically insignificant and the abnormalities resolve after treatment of the hyperthyroidism in the majority of cats. We have also seen cats with hypercalcemia and hyperthyroidism. The Ca abnormalities appear to correct following successful treatment and no other cause for the hypercalcemia has been found. Several recent papers have examined the effects of hyperthyroidism on bone and calcium metabolism in hyperthyroid cats.
One paper investigated the tissue sources of ALP in cats with hyperthyroidism. Alkaline phosphatase as measured in the serum is made up of enzymes that arise from the liver, bone, and intestine as well as other tissues. The source of elevated ALP in hyperthyroid cats has been suggested to be from liver and bone. In this paper isoenzymes of ALP were measured by serum electrophoresis in five normal cats and in 34 cats with hyperthyroidism. Homogenates of kidney, intestine, bone, and liver were subjected to electrophoresis to identify and quantify isoenzymes of alkaline phosphatase. The highest concentration of alkaline phosphatase was found in kidney, followed by intestine, liver, and bone in descending order of concentration. The liver isoenzyme was the only type identified in the serum of normal cats. Serum total ALP activity was elevated in 53% of hyperthyroid cats. Thirty of the 34 hyperthyroid cats had two main isoenzyme bands on electrophoresis corresponding to liver and bone isoenzymes. There was significant correlation between total T4 and total serum ALP concentration and T4 and liver isoenzyme concentration, but not T4 and bone isoenzyme concentrations in hyperthyroid cats. The proportion of bone and liver isoenzyme in sera of hyperthyroid cats varied considerably, with the liver isoenzyme making up 17 to 100% of the ALP and bone 0 to 74% of ALP. The authors concluded that abnormalities in bone and liver account for the elevations in serum ALP found in hyperthyroid cats. This paper is relevant as it points out that clinicians should evaluate thyroid function in an older cats with a high ALP. Although bone disease has not been reported in cats with hyperthyroidism it is likely that the high bone ALP activity is related to increased bone turnover. This was reinforced in 2 papers looking at bone turn over and Ca metabolism in hyperthyroid cats.
The effect of hyperthyroidism on serum markers for increased bone metabolism and turnover was evaluated in 36 cats with elevated serum levels of thyroxine and alkaline phosphatase. Serum was analyzed for total and iCa and phosphorous. Alkaline phosphatase isoenzymes were separated by agarose gel electrophoresis and osteocalcin was measured by radioimmunoassay. Values for hyperthyroid cats were compared with those for healthy cats. Alkaline phosphatase bone isoenzyme was markedly increased in all 36 hyperthyroid cats. Osteocalcin was increased in 44% of the cats. There was no correlation among the magnitude of increase in alkaline phosphatase bone isoenzyme, osteocalcin, and serum thyroxine concentrations. Increased serum phosphorus was found in 35% of the cats. Total calcium was within the reference range in all cats, while 50% of the cats had reduced levels of serum iCa. While clinically insignificant the study provided evidence that bone turnover is increased (as assessed by osteocalcin levels) in hyperthyroid cats. The finding of low iCa in 50% of the cats is interesting and should be repeated in other studies looking at PTH levels in thyrotoxicosis.
One recent paper did just that by looking at parathyroid (PTH) and iCa concentrations in hyperthyroid vs normal cats.They looked at 30 cats with untreated hyperthyroidism and 38 age-matched control cats. The hyperthyroid group of cats were found to have significantly lower blood iCa and plasma creatinine concentrations and significantly higher plasma phosphate and parathyroid hormone concentrations. Hyperparathyroidism occurred in 77 per cent of hyperthyroid cats, with parathyroid hormone concentrations reaching up to 19 times the upper limit of the normal range. The etiology, significance and reversibility of hyperparathyroidism in feline hyperthyroidism remains to be established but could have important implications for both bone strength and renal function. The decreased creatinine concentrations found in the hyperthyroid cats may be due to either increased GFR or decreased muscle mass though this was not evaluated in this study. We will examine the effects of thyrotoxicosis on renal function at the end of this section.
We will continue to learn more about the importance of hyperthyroidism on both clinical and laboratory findings. As many of our hyperthyroid patients are senior and are likely to have concurrent disease it will be very important to look at these interrelations in more detail.
1. Basal T4 concentration
a. As for all endocrine testing, check with the laboratory for normal values and to see if a given assay is validated for the species you are evaluating.
b. In general, normal basal T4 levels support euthyroidism, but low levels do not confirm hypothyroidism as many factors affect basal T4 levels.
c. A low T4 indicates the need for further testing (see below).
2. Basal T3 concentrations
a. Basal levels of little use in discriminating normal from hypothyroid as:
i. Vast majority of T3 is intracellular.
ii. The majority of T3 produced by peripheral deiodination of T4.
3. Factors causing low T4 and T3 in euthyroid animals
a. Hourly fluctuations
b. Fasting over 48 hours
c. Concurrent illness
e. Medications: Glucocorticoids, valium, anticonvulsants, propranolol, many others.
4. Factors causing increased T4 and T3 in euthyroid animals
b. Hourly fluctuations
c. Estrus, pregnancy
d. Medications: Estrogen, progesterone
e. Antithyroid antibodies
TSH stimulation test
1. Designed to eliminate variables affecting basal T3 or T4.
2. Protocol depends on laboratory used. Check first.
3. A common protocol is 0.1 IU TSH/kg IV, serum T4 at time 0 and 6 hours post-TSH.
4. Exogenous thyroid supplementation should be stopped 4 weeks prior to testing.
5. Post-TSH T4 should be within or above normal post-TSH range for laboratory used.
6. Serum T3 response is more variable than T4 and less diagnostic.
7. Human recombinant TSH can be used in the dog although the cost may be prohibitive and the use of assays for free T4 by equilibrium dialysis has limited the use of TSH stimulation testing in dogs.
Interpretation of TSH Stimulation Test
1. With primary hypothyroidism
A. Pre and post-TSH T4 should remain below normal basal T4 range.
2. With secondary and tertiary hypothyroidism
A. Results depend on degree of thyroid atrophy; can resemble normal, sick euthyroid or hypothyroid. May need to treat with TSH for several days to assess the degree of thyroid atrophy.
3. Sick euthyroid
A. Animals with non-thyroidal disease or drug-induced lowering of T4 and T3 will have a blunted response to TSH administration when compared to normal. Differentiating between the sick euthyroid syndrome and hypothyroidism can be difficult and depends on clinical signs, presence of concurrent illness or drug administration, and owners recollection of the onset of signs.
TRH stimulation test
1. To differentiate secondary from tertiary hypothyroidism. Measurement of cTSH concentrations is also recommended.
2. Evaluates release of TSH in response to stimulation by TRH.
3. TSH stimulation test should be performed first to document thyroid responsiveness.
4. Lack of post-TRH T4 increase implies primary or secondary hypothyroidism. If the animal has a normal response to TSH administration, then secondary hypothyroidism is diagnosed.
5. Follow protocol recommended by your lab.
6. Primarily used in evaluating patients with suspected or known abnormalities involving one or multiple pituitary hormones; I.e. pituitary dwarfs or animals with CNS lesions.
Recently, determination of free T4 (fT4) by equilibrium dialysis, has been shown to correlate very well with results of TSH stimulation testing in the diagnosis of canine hypothyroidism. Evaluation of fT4 allows us to assess the biologically active fraction of thyroid hormone and has been shown to be much less affected by non-thyroidal factors (medications, concurrent illness, binding abnormalities, etc). The term "sick euthyroidism" is often used to describe the effect of these various non-thyroidal factors on decreasing TT4 concentrations in the face of normal thyroid function. Although a number of fT4 assays are commercially available, only those that employ a dialysis step are valid in the dog. We have derived the following chart to assist in the diagnosis of canine hypothyroidism utilizing either the TSH stimulation test or evaluation of a resting TT4 and fT4.
Thyroid Function Testing: Interpretation (Refer to your laboratories reference ranges)
TSH Stimulation Testing
Canine TSH Assay
Recently, an advance in the diagnostic approach to hypothyroidism was achieved with the advent of a reliable assay for canine TSH (cTSH). A kit for cTSH (Diagnostic Products Corporation; DPC Inc) is now available and should help in our approach to the patient with suspected hypothyroidism. A cTSH together with a free T4 by dialysis should provide the most relevant information with respect to thyroid function. A patient with hypothyroidism should have an elevated cTSH in conjunction with a decreased fT4. However, with the current cTSH assay, up to 25% of patients with confirmed hypothyroidism have a cTSH concentration within the normal range.
Thyroid Panel (fT4 and cTSH)
Antithyroglobulin and anti-T3 and anti-T3 autoantibody testing:
The presence of antithyroglobulin antibodies indicates the presence of lymphocytic thyroiditis. Occasionally, these animals may also have anti-T3 and anti-T4 (rare) autoantibodies. The presence of thyroiditis does not equal a diagnosis of hypothyroidism. Animals with thyroiditis likely will become hypothyroid in the future but the decision on whether to supplement with thyroid hormone should be based on the presence of clinical signs and abnormal function tests (low TT4, low fT4ED and an elevated cTSH level). Animals with thyroiditis should not be used for breeding and this test in now included as part of the OFA thyroid registry. The OFA thyroid panel consists of a TT4, fT4ED, cTSH and antithyroglobulin antibody. To receive a registry number animals must be at least 1 year of age and have normal test results.
Trial therapy with thyroxine
1. Has been advocated as a diagnostic aid for hypothyroidism
2. Response to therapy is nonspecific however and normal animals may show some clinical effect due to the anabolic effects of thyroxine.
3. Indiscriminate therapy with thyroxine, while it may not be harmful, is not cost-effective and may lead to a delay in obtaining a correct diagnosis and instituting proper therapy. Supplementing a sick, euthyroid animal may be detrimental.
As can be seen from the preceding discussion the diagnosis of hypothyroidism depends on a combination of clinical signs, results of routine laboratory tests, and tests of thyroid function. Measurement of fT4 by equlibrium dialysis together with a cTSH provides the most accurate information regarding thyroid function.