More than just T₄: diagnostic testing for hyperthyroidism in cats

Abstract

Clinical challenges: In older cats presenting with clinical features of hyperthyroidism, confirmation of a diagnosis of thyroid disease is usually straightforward. However, the potential for false-negative and false-positive results exists with all thyroid function tests (especially in the context of routine screening of asymptomatic cats) and leads to clinical dilemmas. For example, a high serum T₄ value may be found in a cat that lacks clinical signs of hyperthyroidism, or hyperthyroidism may be suspected in a cat with normal total T₄ concentrations.

Practical relevance: To avoid unnecessary treatment and potentially adverse effects in a euthyroid cat, thyroid function tests must always be interpreted in the light of the cat's history, clinical signs, physical examination findings and other laboratory findings.

Evidence base: In this article the author reviews the use of commonly recommended thyroid function tests, focusing on clinical scenarios that present diagnostic difficulties. In doing so, he draws on the veterinary and comparative literature, his own clinical experience, and data, unpublished to date, obtained from a series of 100 hyperthyroid cats consecutively diagnosed at his clinic.

Figure 1

Thyroid palpation techniques: (a) the ‘classic’ technique; (b,c) the second (Norsworthy) technique. In (b) the author is palpating for the right thyroid lobe and in (c) he is searching for the left thyroid lobe

Figure 2

Scatter plots of total T₄ concentrations measured by CEIA and EIA in 100 cats (60 with untreated hyperthyroidism, 40 being monitored after radioiodine treatment). The cats ranged in age from 9–19 years (median 13 years); almost all were of mixed breeding (domestic shorthair or domestic longhair), with 47 castrated males and 53 spayed females. In each graph, each point represents the measurement of total T₄ concentration by the two assays, and the line represents the best fit of the data for linear regression analysis. (a) Scatter plots for all 100 cats. The correlation between the two methods was 0.97 for the 100 serum samples. (b) Scatter plots for 66 cats with values <100 nmol/l, as measured by CEIA. The correlation between the two methods was 0.93 for this group of 66 serum samples. Shaded areas indicate reference interval limits for the two T₄ methods. The cats with discordant T₄ results are shown in red

Figure 3

Box plots of serum concentrations of total T₄, total T₃, free T₄ and TSH in 100 cats with untreated hyperthyroidism. These hyperthyroid cats were consecutively diagnosed and ranged in age from 8–20 years (median 13 years); almost all were of mixed breeding (domestic shorthair or domestic longhair), with 45 castrated males and 55 spayed females. (a) Serum total T₄ concentrations, determined by CEIA. (b) Serum total T₃ concentrations, determined by CEIA. (c) Serum free T₄ concentrations, determined by equilibrium dialysis. (d) Serum TSH, determined by canine TSH CIEA. In each graph, the box represents the interquartile range (ie, 25th to 75th percentile range or the middle half of the data). The horizontal bar in the box represents the median value. For each box plot, the T-bars represent the main body of data, which in most instances is equal to the range. Outlying data points are represented by open circles. The shaded areas indicate the reference interval for each hormone. For the TSH assay, the detection limit (not indicated) is 0.03 ng/ml

Figure 4

(a) Thyroid scintigraphy in a euthyroid cat. In normal cats, the thyroid gland appears on thyroid scans as two well-defined, focal (ovoid) areas of radionuclide accumulation in the cranial to middle cervical region. The two thyroid lobes are symmetrical in size and shape and are located side by side. Activity in the normal thyroid closely approximates activity in the salivary glands, with an expected ‘brightness ratio’ of 1:1. (b,c) Thyroid scintigraphy in two cats with early hyperthyroidism. Both cats had borderline high serum concentrations of total T₄ with slightly high free T₄ values. The cat in (b) has a unilateral left thyroid adenoma, whereas the cat in (c) has bilateral adenomas. In both cases, the uptake of the radionuclide by the functional thyroid adenoma(s) is higher than the uptake by the cat’s salivary tissue. For both cats, a high thyroid:salivary ratio was calculated, which was diagnostic for hyperthyroidism