Thyroid testing is controversial in birds. The thyroid test, T4 that is routinely run on birds is a RIA test, which is primarily used for canine and feline testing. Unfortunately, since the normal thyroid levels for avian species are so much lower than those found in dogs and cats; the test is not going to be the most accurate for birds.
An experimental test has been developed that is not commercially available yet for avian thyroid testing. Until that methodology becomes available, we must make do with the testing at hand.
The RIA test that is currently being used for thyroid testing is accurate down to approximately 0.5 ug/dl. At this point it begins to lose its linearity and accuracy, down to 0.2 ug/dl. Once the value drops below 0.2 ug/dl, the value becomes unreliable.
Adding to the diagnostic dilemma is the fact that the normal range of T4 varies considerably between the different species of birds, even within birds of the same Genus.
Hypothyroidism should never be diagnosed based on ONE low T4 value. Very few documented cases of hypothyroidism have been diagnosed in birds. To accurately diagnose hypothyroidism, a TStim should be performed. However, the necessary hormone, THS, is a human product that costs approximately $1,000 for a box containing two vials (Genzyme). A bovine derived THS is available, but it is NOT produced for use in animals, and is made for experimental use only. With concerns about BSE, I would not recommend using this product in avian patients.
Signs of hypothyroidism in birds may include obesity, hypercholesterolemia, delayed molting (and dull, depigmenting feathers), hyperlipidemia, lipomas, sterile granulomas on the pads of the feet, disseminated xanthomas and one reported case in an owl with an unusual periosteal reaction.
If a diagnosis of hypothyroidism is made in a psittacine, L-thyroxine is the supplement of choice. However, I recommend that you consult with a compounding pharmacist, as the hormone is not stable in water and must be compounded in oil, which should provide a six month shelf-life. It is dosed at 0.02-0.04 mg/kg q 24 hr. Another reference gives a dosage of 0.02 mg/kg PO q 12-24 hr. If thyroid supplementation is provided to a bird that is euthyroid, this may result in hypertrophic cardiomyopathy and heart failure.
To perform a TStim test, 1.0-2.0 IU of TSH is administered IM. Pre, 4 hour, and 6 hour post samples are compared. The post T4 value should at least double the pre T4 samples. A lack of response to the TStim test should provide evidence of hypothyroidism. In addition to history, signalment and clinical signs, feather follicle biopsy and other diagnostics may prove helpful in determining a diagnosis.
If pruritis is a clinical sign, consider the possibility of giardiasis. Many feather-picking cockatiels are harboring protozoa that can cause severe pruritis. However, many of the larger species of psittacine may also show pruritis if harboring Giardia, as well. The most effective treatment for most protozoal infections in birds is ronidazole, available from Global Pigeon Supplies, 912-356-1320. The dose is 1 tsp. per gallon of drinking water for 6-10 days or 6-10 mg/kg PO q 24 hr. for 6-10 days.
Hopefully, newer, more sensitive testing for the diagnosis of avian hypothyroidism will be available in the near future. Until more sensitive testing is available, please be very careful about diagnosing hypothyroidism and treating suspected cases.
Copyright © 2006 Margaret A. Wissman, D.V.M., D.A.B.V.P.
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