Prevalence and degree of thyroid pathology in hyperthyroid cats increases with disease duration: a cross-sectional analysis of 2096 cats referred for radioiodine therapy

Abstract

Objectives: Hyperthyroidism is common in cats, but there are no reports that evaluate its severity or underlying thyroid tumor disease based on disease duration (ie, time from original diagnosis). The objective of this study was to compare serum thyroxine (T4) concentrations and thyroid scintigraphic characteristics of cats referred for radioiodine treatment based on disease duration.

Methods: This was a cross-sectional study of 2096 cats with hyperthyroidism. Cats were divided into five groups based on time from diagnosis: ⩽1 year (n = 1773); >1-2 years (n = 169); >2-3 years (n = 88); >3-4 years (n = 35); and >4-6.1 years (n = 31). Methimazole, administered to 996 (47.5%) cats, was stopped at least 1 week prior to examination to allow for serum T4 testing. Each thyroid scintiscan was evaluated for pattern (unilateral, bilateral, multifocal), location (cervical, thoracic inlet, chest) and size (small, medium, large, huge) of the thyroid tumor, as well as features suggesting malignancy.

Results: Median serum T4 concentration increased with increasing disease duration from 100 nmol/l (⩽1 year) to 315 nmol/l (>4-6.1 years) (P <0.001). Prevalence of unilateral thyroid disease decreased, whereas multifocal disease (three or more tumor nodules) increased (P <0.001) with increasing disease duration. Median tumor volume in the five groups increased from 1.6 cm(3) (⩽1 year) to 6.4 cm(3) (>4-6.1 years). Prevalence of large (4-8 cm(3)) and huge (>8 cm(3)) thyroid tumors increased from 5.1% (⩽1 year) to 88.6% (>4-6.1 years), while the prevalence of intrathoracic tumor tissue increased from 3.4% (⩽1 year) to 32.3% (>4-6.1 years). Prevalence of suspected thyroid carcinoma (characterized by severe hyperthyroidism; huge, intrathoracic, multifocal tumors; refractory to methimazole treatment) increased with increasing disease duration from 0.4% (⩽1 year) to 19.3% (>4-6.1 years).

Conclusions and relevance: Our results indicate that the prevalence of severe hyperthyroidism, large thyroid tumors, multifocal disease, intrathoracic thyroid masses and suspected malignant disease all increase with disease duration in cats referred for radioiodine therapy.

Figure 1

Box plots of (a) serum thyroxine (T4) concentrations and (b) thyroid to salivary (T/S) ratios in 2096 cats with hyperthyroidism, divided into the following five groups based on the time interval from diagnosis of hyperthyroidism to thyroid scintigraphy: group 1 (n = 1773) ⩽1 year; group 2 (n = 169) >1–2 years; group 3 (n = 88) >2–3 years; group 4 (n = 35) >3–4 years; and group 5 (31 cats) >4–6.1 years. In each graph, the box represents the interquartile range (ie, 25th–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 2.5–97.5th percentile range

Figure 2

Thyroid scintigrams illustrating the major patterns of thyroid disease with one to six areas of sodium pertechnetate uptake in the 2096 cats. (a) One nodule (unilateral disease). (b) Two nodules (bilateral cervical disease). (C) Three nodules (bilateral disease with third ectopic midline nodule in thorax). (d) Six nodules (multifocal disease resulting from suspected thyroid carcinoma)

Figure 3

Bar graphs depicting the percentage of hyperthyroid cats with unilateral, bilateral and multifocal (three or more nodules) thyroid disease, in each of five groups, based on the time from diagnosis: group 1, ⩽1 year; group 2, >1–2 years; group 3, >2–3 years; group 4, >3–4 years; and group 5, >4–6.1 years

Figure 4

Bar graphs depicting the percentage of cats with thyroid tumor nodules in the cervical, thoracic inlet and intrathoracic locations in each of the five groups, based on the time from diagnosis: group 1, ⩽1 year; group 2, >1–2 years; group 3, >2–3 years; group 4, >3–4 years; and group 5, >4–6.1 years

Figure 5

Box plots of the calculated volume of thyroid tumor tissue in 2096 cats with hyperthyroidism, divided into the following five groups based on the time from diagnosis: group 1, ⩽1 year; group 2, >1–2 years; group 3, >2–3 years; group 4, >3–4 years; and group 5, >4–6.1 years

Figure 6

Bar graphs depicting the percentage of hyperthyroid cats with small, medium, large and huge thyroid tumors in each of five groups, based on the time from diagnosis: group 1, ⩽1 year; group 2, >1–2 years; group 3, >2–3 years; group 4, >3–4 years; and group 5, >4–6.1 years

Figure 7

Bar graphs depicting the percentage of hyperthyroid cats with ectopic thyroid tumor tissue, suspected thyroid carcinoma, or SHIM-RAD tumors in each of five groups, based on the time from diagnosis: group 1, ⩽1 year; group 2, >1–2 years; group 3, >2–3 years; group 4 >3–4 years; and group 5, >4–6.1 years. Cats in the SHIM-RAD subgroup had the following five features: (1) severe hyperthyroidism (serum thyroxine >300 nmol/l); (2) huge tumors; (3) intrathoracic tumor location; (4) multifocal distribution of radionuclide uptake; and (5) refractory to treatment with antithyroid drugs