Thyroid diseases are rarely diagnosed in pet rabbits and very little is known about the effects of systemic illnesses on thyroid hormone levels in this species. This brief communication compares the results of total thyroxine (T4, n = 44) and free T4 (n = 26) measurements in sick rabbits with total T4 (n = 56) and free T4 (n = 28) measurements from apparently healthy animals. Mean values of total and free T4 were significantly lower in sick animals, indicating nonthyroidal illness syndrome in the pet rabbit. Unlike in the dog, total and free T4 were both decreased to a similar degree in diseased rabbits. Rabbits with fractures and presumed obstructive ileus had particularly low total T4 levels. Total T4 values of rabbits suffering from urinary tract diseases were affected to a lesser extent. The effect of different diseases on free T4 values could not be evaluated on account of the small sample size. This study suggests that, similar to other species, total and free T4 measurement alone might not be sufficient to diagnose hypothyroidism in the pet rabbit due to the existence of nonthyroidal illness syndrome.
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