Understanding thyroid testing in the dog and cat (Proceedings)
The hypothalamus secretes thyrotropin-releasing hormone in response to nervous stimuli. The TRH then stimulates the pituitary gland to secrete thyrotropin-secreting hormone (TSH). The follicular cells of the thyroid gland then release thyroid hormone in response to TSH. Thyroxine (T4) and tri-iodothyronine (T3) provide negative feedback primarily at the level of the pituitary. Thyroxine is the primary iodothyronine, or thyroid hormone, released from the thyroid gland. Tri-iodothyronine is also released in a much smaller quantity. Thyroid hormones consist of both free and bound fractions. Ninety-nine percent are bound to albumin, prealbumin and thyroid hormone-binding globulin. It is the free fraction of T4 that can enter cells where it is converted to the biologically active portion, T3 as well as rT3.
Total serum thyroxine (TT4) consists of free and unbound fractions and is usually measured with human radioimmunoassay's (RIA) and ELISA's validated for the dog and cat. There are in-house ELISA's that measure TT4 but there are conflicting results regarding their accuracy. Anti-thyroxine antibodies can artificially increase TT4 levels with some assays. Non-thyroidal illness and certain drugs can falsely decrease the TT4. Total thyroxine is commonly used as a screening test for dogs with hypothyroidism and cats with hyperthyroidism.
Free Total Thyroxine
In both dogs and cats, non-thyroidal illness can falsely decrease TT4 because total serum thyroxine can be affected by the quantity of carrier proteins, alterations in metabolism, the ability to transport thyroxine into cells and the binding of T4 within the cells. Anti-thyroxine antibodies occur in about 2% of hypothyroid dogs and can falsely elevate TT4. It is also possible that cats with early hyperthyroidism may have TT4 levels that wax and wane despite clinical signs and a normal TT4 may be found in a hyperthyroid cat. For these reasons, the unbound or free fraction of TT4 is often measured in dogs and cats in which TT4 is non-diagnostic. Free T4 is not altered by those factors that affect TT4 in non-thyroidal illness. Tests for fT4 using equilibrium dialysis (ED) are recommended. Equilibrium dialysis also eliminates anti-thyroxine antibodies. Because ED is time-consuming and expensive, human radioimmunoassay's (RIA) have been used in dogs and cats. These RIA's consistently measure a lower fT4 than ED and are of no diagnostic value over TT4. Because fT4 is a more sensitive test in the dog and cat it seems logical to use it as a screening test but unfortunately a few euthyroid dogs have a low fT4 and some euthyorid cats have an increased fT4. So although the fT4 is more sensitive it is not a perfect test. Free T4 is commonly used in dogs and cats when the TT4 is non-diagnostic.
The pituitary secretes thyroid-stimulating hormone (TSH) in response to thyrotropin-releasing hormone (TRH) from the hypothalamus. Thyroxine provides negative feedback so in dogs with primary hypothyroidism resulting in a low T4, the endogenous TSH should be increased. Unfortunately the TSH of many hypothyroid dogs is within the reference range. This may be due to fluctuations in TSH, the effects of drugs or concurrent disease on TSH production and the presence of secondary (pituitary) or tertiary (hypothalamus) hypothyroidism. The TSH might also be increased in euthyroid dogs which may be due to early hypothyroidism, recovery from non-thyroidal illness and certain drugs. Endogenous TSH is a less sensitive test than either TT4 or fT4. Endogenous TSH is most often used concurrently with TT4 and fT4 for diagnosis of hypothyroidism in the dog. Currently there are no assays available to measure TSH in the cat.
TSH Response Test
Unfortunately, in some dogs it may be difficult to determine if they are hypothyroid with the tests mentioned above. In these cases a baseline TT4 can be measured and then human recombinant TSH (25 to 100 µg) administered IV and another TT4 measured at 6 hours. In the presence of a normal thyroid stimulation of TT4 should occur but in primary hypothyroidism little stimulation should occur and, in fact, both samples may be below the reference range.
TRH Response Test
The TRH response test can be used to diagnose hypothyroidism in dogs and hyperthyroidism in cats. Thyrotropin-releasing hormone is released by the hypothalamus and increases the production of TSH by the pituitary and subsequent T3, T4 and rT3 from the thyroid gland. A baseline TT4 is measured, thyrotropin-releasing hormone administered IV (0.01 mg/kg) and serum TT4 measured 4 hours later. In a normal dog or cat TT4 should increase twofold or more but hypothyroid dogs remain below the reference range. Hyperthyroid cats stimulate less than 50% above baseline because the chronically elevated T4 and T3 suppress TSH secretion so the pituitary is less responsive to exogenous TRH.
T3 Suppression Test
If the diagnosis of hyperthyroidism can not be made with the TT4 and fT4, the T3 suppression test can be used. Thyroid hormone (T3, levothyroxine) is administered to suppress TSH and subsequent T4 production. Baseline serum TT3 and TT4 are measured and then the cat is given 25 µg liothyronine three times a day for 2 days. A final dose is given the morning of the test and TT4 and TT3 are measured 2 to 4 hours after the last dose. Normal cats should have a decreased TT4 but because of the autonomous function of the thyroid gland, TT4 in hyperthyroid cats decreases little if at all. Serum TT3 is measured to evaluate successful administration of T3 and should increase.
Antithyroglobulin, Anti-T3 and Anti-T4 Antibodies
Most cases of hypothyroidism in the dog are due to lymphocytic thyroiditis or follicular atrophy. Follicular atrophy may be a separate entity or the result of lymphocytic thyroiditis. Antithyroglobulin antibodies are found in up to 50% of dogs with hypothyroidism. Anti-T3 and anti-T4 antibodies are less common but thought to form against thyroglobulin as well. Unfortunately, these antibodies may be present prior to the development of clinical hypothyroidism so their presence does not necessarily indicate treatment. This is why antithyroglobulin antibodies are used by breeders as a screening test in younger animals.
In some cases in which TT4 and FT4 are inconclusive, radionuclide imaging can be used. The thyroid gland concentrates certain substances such as radioactive iodines and technetium. Radioactive iodines can be used but are less practical because of the greater radiation exposure to humans, longer elimination half lives and longer time to acquire images. Pertechnetate is structurally similar to iodine so it is concentrated in the thyroid gland like iodine. It is inexpensive, has a shorter half-life, only emits low energy gamma particles and imaging can begin 20 minutes after IV administration. Technetium 99m, and the other radionuclides, are given intravenously and the radioactivity measured by a camera and recorded on film. Radioactive particles concentrate in the thyroid, salivary glands and gastric mucosa of the normal cat. In a normal dog and cat the radio intensity should be 1:1 between the salivary glands and thyroids. The salivary glands and thyroid lobes should also be similar in size. In the hypothyroid dog decreased uptake is seen. There have been a few cases of active thyroiditis with increased uptake in the dog. Scintigraphy can also be used to distinguish types of congenital hypothyroidism since in some pertechnetate will be increased and in others decreased. In the hyperthyroid cat the thyroid and any ectopic tissue will be more radio intense. There are also other types of masses that can occur in the cervical region that might be confused with a thyroid tumor. These should not concentrate radionuclide.
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