Total T and free T
In the initial diagnostic approach to a cat with hyperthyroidism, the best test is still total T4 measurement. This test is simple and inexpensive and provides the correct diagnosis in most cases. However, we now are often
faced with diagnosing or confirming hyperthyroidism in cats that are asymptomatic, that have only mild clinical signs, or
that have concurrent illness that may skew laboratory assessment of thyroid function. These cases can be challenging, but
recent research indicates that measuring free T4 by equilibrium dialysis is the next logical diagnostic step. This approach is likely to eliminate the need for additional
expensive or problematic tests such as thyrotropin-releasing hormone stimulation and T3 suppression.
Two studies have assessed the value of free T4 measurement by equilibrium dialysis in diagnosing feline hyperthyroidism and have evaluated the effects of nonthyroidal illness
on thyroid function.4,5 As is the case in dogs, euthyroid cats with nonthyroidal illness may have a decrease in total T4 concentrations that probably results from protein-binding abnormalities. In a study of 98 cats with nonthyroidal illness
and 50 normal control cats, thyroid function was assessed by measuring total T4 and free T4 concentrations.4 Serum total T4 concentrations were measured by radioimmunoassay, and free T4 concentrations were measured by equilibrium dialysis. Serum total T4 concentrations were significantly lower in sick cats (1.33 ± 0.63 µg/dl [17.18 ± 8.14 nmol/L]; reference range = 0.8 to 4
µg/dl) than in healthy cats (2 ± 0.59 µg/dl [26 ± 7.62 nmol/L]). Serum total T4 concentrations were inversely correlated with mortality. No significant difference was noted between the serum free T4 concentrations in sick cats (2.15 ± 1.05 µg/dl [27.7 ± 13.53 pmol/L]; reference range = 0.78 to 3.9 µg/dl [10 to 50 pmol/L])
and in healthy cats (1.93 ± 0.65 µg/dl [24.79 ± 8.33 pmol/L]). Twelve percent of sick euthyroid cats had increased serum free
T4 concentrations. This study showed that, as is the case in dogs and people, euthyroidism is maintained in sick cats, despite
low serum total T4 concentrations. Also the serum total T4 concentration is a valuable prognostic indicator because it appears to be an excellent predictor of mortality. Lastly, and
perhaps more importantly with respect to diagnosing hyperthyroidism, some euthyroid older cats have elevated free T4 concentrations measured by equilibrium dialysis. Thus, initially measuring free T4 by equilibrium dialysis as a screening test for hyperthyroidism in older cats can lead to false positive results.