Thyroid function tests in dogs using radioimmunoassay kits

Thyroid function has been assessed using radioimmunoassay kits for total and free thyroxine (T4). A radioimmunoassay kit for total T4, designed for human samples, was unable to detect total T4 in 35 per cent of normal canine serum tested, due to the kit being designed to measure total T4 over the normal human range which is four times the canine normal range. The canine normal range for free T4 (7.5–50 pmol/1) was found to be of the same order as the human range and therefore a free T4 kit was suitable for measuring the free hormone in dogs. In response to thyroid stimulating hormone in normal dogs, total T4 rose 6.2 times whereas free T4 rose only 2.6 times. Hypothyroid animals could be distinguished from normal dogs by free T4 assay whereas they were indistinguishable from many normal dogs by total T4 assay. Response to thyroid stimulating hormone was used to confirm hypothyroidism, but also revealed one animal with normal total and free T4 but which had a negligible response on stimulation.