Confusion over the diagnostic utility of tree T4 by equilibrium dialysis (FT4ED) had lead to misdiagnosis of hyperthyroidism in the cat. Use of FT4ED alone as a diagnostic aid (without evaluating a concurrent T4), or as a screening test in healthy cats, has lead to more cats being diagnosed as hyperthyroid than actually have the disorder.

Free T4 is the non-protein bound, physiologically active form of T4 and therefore should be the best marker of thyroid status in dogs and cats. Indeed, in cats with overt hyperthyroidism (consistent clinical signs and T4 > 4 meg/dL), free T4 ED values are elevated. In cats with early or mild hyperthyroidism (more mild signs of disease and T4 in the upper 1/2 of normal reference range), the FT4ED is usually elevated.

However in 5-10% of cats with nonthyroidal illness (e.g., chronic renal failure, IBD, liver disease, neoplasia) the FT4ED can be elevated for reasons that are unclear. These cats may have signs consistent with hyperthyroidism, (e.g., weight loss, vomiting) but there is typically no palpable thyroid nodule(s), and T4 values are typically less than 2 µg/dL.

Case example: A 15-year-old castrated male cat was presented for polyuria, polydipsia, and mild weight loss. The serum chemistry profile showed elevated BUN and creatinine (40 mg/dL and 2.7 mg/dL, respectively). Urine sp. gr. was 1.010, FT4ED was 70 nmol/L (normal range 10-50 nmol/L), and T4 was 1.0 µg/dL (normal range (1.8-4 µg/dL). Assessment: Chronic renal failure with spurious elevation of FT4ED. This cat was not hyperthyroid; the diagnostic key was the T4 in lower half of normal reference range along with abnormal renal parameters.

Conclusion: An FT4ED concentration in cats has little clinical meaning without evaluating the T4 concurrently, as well as results of the history, physical exam, and routine bloodwork.