Times change and my website needed to change too. To see the 2020 update of this page click this link
To see what normal blood and urine values are, go here
For an explanation of causes of most abnormal blood and urine tests, go here
To see how tests are grouped, go here
Low thyroid issues or hypothyroidism is exceedingly rare in cats. Cats are much more likely to have over-active thyroid glands or hyperthyroidism
The free T4 (fT4) test is a more sensitive test than the total T4 (tT4) test. But it is less specific than tT4 - at least for diagnosing feline hyperthyroidism. That is, using the free T4 test as an initial screening test in cats can result in more false-positives than the total T4 test. Veterinarians generally reserve the free T4 test for pets that have other signs that make them suspicious that a pet is hyperthyroid or hypothyroid but where the total T4 test is borderline and hard to interpret (about 10-30% of hyperthyroid cats tested [ref]). In others, problems such as kidney disease can give false T4 level depression. (ref) Free T4 seems to be more helpful in dogs - particularly when a case of low thyroid function (hypothyroidism) is suspected but the pet's total T4 results were indecisive. (ref)
In the case of cats suspected of being hyperthyroid, when both total T4 and free T4 are on the high end of the lab’s reference range and the cat has other symptoms of the problem, veterinarians can be more assured of their diagnosis. A low TSH is an addition aid in confirming the diagnosis of hyperthyroidism in cats. So is a confirmatory total T4 test in 3 weeks or so. Advanced university veterinary centers can also confirm the diagnosis using thyroid scintigraphy. (ref)
Your pet’s two thyroid hormones, T3 (triiodothyronine) and T4 (thyroxin) are responsible for regulating its metabolic rate (the amount of energy its body consumes).
Most of your pet’s thyroid hormone is attached to blood transport proteins and not available for its cells to use. It is only the small amount of unbound or free T4 that is active. So measuring that, rather than the total, gives your veterinarian that information. (It is often referred to on the pet’s lab report as T4 by equilibrium dialysis).
It is quite common for any long-term health problem to lower your dog or cat's total T4 levels. But free T4 levels are often less affected in those situations (although they can be lowered a bit as well). Veterinarians call that the “sick euthyroid syndrome” or “NTI”. It refers to the fact that I just mentioned that about any health problem or syndrome can cause low T4 levels - not just those related to the pet’s thyroid gland (ie hypothyroidism and hyperthyroidism).
In true hypothyroidism in dogs, free T4 levels are often lower than normal before T4 and T3 levels begin to go down.
Autoimmune thyroiditis, an inherited form of hypothyroidism in dogs, will also produce low FT4 levels, as can a number of genetic abnormalities. (ref)
Sighthounds (like greyhounds) have naturally lower T4 levels. Just because their Thyroid hormone tests come back as low does not mean they are hypothyroid.
Certain medications (eg phenobarbital [ref], sulfonamides, and corticosteroids) can lower your pet's FT4 readings.
Over 90% of the cases of high T4 (hyperthyroidism) veterinarians see are in mature-to-elderly cats. The problem is almost never seen in dogs.
A true high Free T4 level in dogs is very rare - so recheck T4, Free T4 and TSH levels in 2-3 weeks – (earlier if the dog has obvious serious health problems or is loosing weight). Low free T4 in cats is also quite rare. So repeat those tests in your cat as well. TSH stimulation test (or T3 suppression test), thyroglobulin autoantibody (=TgAA false positive and negative tests do occur)
It is unwise to base a diagnosis of hypothyroidism on a single blood sample.
Sometimes, the best option available when thyroid test results remain foggy and difficult for your veterinarian to interpret is a trial period on a moderate dose of thyroid hormone replacement.