Ron Hines DVM PhD
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
Meds that might interfere with thyroid gland test results here
Does 1 low thyroid test result = hypothyroidism? here
Your Pet’s Total T4 Level = T4 or Total Thyroxine
Your dog or cat’s T4 (Total T4) is a useful screening test to detect an under-active or an over-active thyroid gland (hypothyroidism/ hyperthyroidism ). In dogs, an under-active thyroid gland is most common (hypothyroidism). In cats an over-active thyroid gland is most common (hyperthyroidism). Total T4 levels are considerably more accurate in diagnosing an overly active thyroid gland in your cat than they are in diagnosing an under-active thyroid gland in your dog.
The T4 test is not the same thing as a the Free T4 test. The free T4 test, is performed using a technique called equilibrium dialysis. It measures the amount of T4 that is actually free and available (biologically active) in your pet’s system and not tied up (attached or bound to) proteins present in your dog and cat’s blood. The rapid tests that your veterinarian performs in the office (such as the Idexx SNAP® T4) determines your pets total T4 level. That, or a T4 level determined by a national laboratory, are acceptable screening tests to access the likelihood of thyroid gland disease in your cat or dog. But when the results are not clear, are borderline or do not fit well with the symptoms your pet is experiencing, your veterinarian will probably order other more advanced tests, including a Free T4 test. About 99% of the T4 in your dog and cat’s blood is bound to its blood proteins and not available to be converted into T3 which is the active form of the hormone that cells actually utilize to adjust their metabolic rate. When your pet has beginning thyroid gland issues, the Free T4 test will usually pick up the problem before the other tests do.
In-office T4 tests are also an economical way to properly adjust your pet’s dose of thyroid medication as its treatment progresses.
Although in-office Total T4 tests to check your pet’s thyroid function are fast and convenient for you and your veterinarian, they sacrifice some reliability for that convenience.
Sometimes the only way to confirm those difficult cases is to simply give a thyroid supplement medication for a few months and see what happens (under your veterinarian’s supervision of course). If an improvement in a dog’s attitude and energy level or a lower blood lipid level (cholesterol, triglycerides) is seen within the first few weeks or a return to a normal hair coat after a longer period, hypothyroidism could have been the underlying cause. But those dogs need follow up monitoring. There are risks in giving thyroid medications to pets that do not need them – probably the same risks as seen in humans under the same situations. (ref)
There are two test procedures used to measure T4, the RIA and the ELISA method. In borderline situations they can yield different results – although that appears to be chiefly due to what different testing laboratories consider normal T-4 levels to be. (ref) (ref)
In those borderline situations especially, do not compare your pet’s results from one lab with results from another – run them all through the same laboratory.
Reasons Why Your Cat Or Perhaps Even Your Dog’s T4 Level Could Be High:
If you own a cat that has the symptoms of hyperthyroidism and it’s T4 level is elevated, it is most likely hyperthyroid. Hyperthyroidism is quite rare in dogs. So high results in a dog need to be confirmed with a thyroid panel.
Pets less than a year old tend to to have a bit higher T4 values. As pets age, their T4 levels naturally decrease. So what might be considered normal in a young adult pet would not necessarily be normal in a geriatric dog or cat. In the very rare instances when dogs have abnormally high T4 levels, the cause is a tumor(s) present in their thyroid gland. Some veterinarians believe that pets that are in heat or pregnant can have slightly higher T4 levels. I don’t know of any scientific work has been done to confirm that. But we do know that in pregnant women, T4 and T3 levels are about 1.5 times higher than normal while free T3 and free T4 are often lower than normal.
Reasons Why Your Dog Or Perhaps Even Your Cat’s T4 Level Might Be Low:
Decreased T4 levels in dogs can mean that your pet is hypothyroid. But as I mentioned before, a non-thyroid illness depressing T4 levels has to be ruled out before that diagnosis is made. Many pet owners associate chubby cats (and people) with sluggish thyroid gland function (low T4). Hypothyroidism is not why most cats (& people) become overweight. It is because we or our furry companions eat too much and exercise too little. True low T4 levels in cats are quite rare. When they do occur, it is usually in a kitten or puppy that was born a dwarf. (ref1, ref2) We are making some progress in understanding the genetics that are involved in hypothyroidism in dogs. (ref)
As I mentioned, a wide variety of chronic diseases that have nothing directly to do with primary disease of the thyroid gland will also cause depressed T4 levels. So your vet will probably want to perform a complete thyroid panel, and do a thorough general health evaluation on your dog or cat before making that decision. Many medications can also lower your pet’s T4 levels. Phenobarbital, given for epilepsy is occasionally one of them. (ref)
Certain breeds of dogs – those that were bred to run fast and track animals with their eyesight rather than their nose have lower T4 levels (about 1/3 – 1/2 lower) naturally. That group are called sight hounds (greyhounds, salukis, Afghan hounds, whippets, borzois, Irish wolfhounds, pharaoh hounds, ibizan hounds, etc.)