# Thyroid Scintigraphy Findings in 234 Hyperthyroid Cats Before and After Radioiodine Treatment

## TL;DR

This study evaluates thyroid scans in 234 hyperthyroid cats before and after radioiodine treatment to assess treatment outcomes and thyroid function.

## Contribution

The study provides the first comprehensive evaluation of thyroid scintigraphy for monitoring treatment success in hyperthyroid cats after radioiodine therapy.

## Key findings

- Most cats (70.5%) returned to normal thyroid function after treatment, while 23.1% had mild underactivity.
- Thyroid scans showed reduced tumor activity in all cats, but could not reliably distinguish between normal and mildly underactive thyroid function.
- Persistent hot nodules occurred in 11% of cats, but all had normal thyroid function.

## Abstract

Thyroid scintigraphy is an important tool for diagnosing and staging hyperthyroidism in cats, but it has not been evaluated for treatment success after radioiodine. In a study of 234 hyperthyroid cats, thyroid scans were performed before and six months after treatment. Based on thyroid blood tests, 165 cats (70.5%) returned to normal thyroid function, 54 cats (23.1%) had mild thyroid underactivity (subclinical hypothyroidism), and 15 cats (6.4%) developed more severe hypothyroidism. With follow-up scintigraphy, all cats showed a reduction in the size and activity of the overactive (“hot”) thyroid tumor tissue. In cats with one-sided thyroid disease, the opposite thyroid lobe regained function in 60 of 99 cats (61%). In cats with both thyroid lobes affected, 108 of 135 (80%) maintained thyroid function in both lobes. Persistent “hot” nodules occurred in 26 (11%) cats, but all had normal thyroid function. In contrast, 24 cats (10.4%) had little to no remaining thyroid tissue on follow-up scans, and most of these cats (17/24) had become hypothyroid. Although thyroid scans can help monitor the effectiveness of treatment and confirm destruction of the cat’s thyroid tumor, their ability to distinguish between normal and mildly underactive thyroid function is limited.

Thyroid scintigraphy is a key tool for diagnosing and staging hyperthyroidism in cats, but follow-up scintigraphic studies after radioiodine treatment are limited. This multicentric study evaluated 99mTc-pertechnetate scintigraphy findings in 234 hyperthyroid cats before and 6 months after radioiodine treatment. Based on serum T4 and TSH concentrations, 165 (70.5%) became euthyroid, 54 (23.1%) had subclinical hypothyroidism, and 15 (6.4%) developed overt hypothyroidism. On post-treatment scintigraphy, all cats showed reduced size and radionuclide uptake of “hot” thyroid nodules. Of 99 cats with unilateral nodules, 60 (61%) recovered function in the contralateral lobe. Among 135 cats with bilateral nodules, both lobes remained visible in 108 (80%). Persistent “hot” nodules with high thyroid/salivary (T/S) ratios or thyroidal pertechnetate uptake (TcTU) occurred in 26 (11%) cats, all of which were euthyroid. Conversely, 24 (10.4%) cats had minimal or absent thyroid tissue with 17 (71%) being hypothyroid, but seven (29%) were euthyroid. As a diagnostic test for iatrogenic hypothyroidism, TcTU showed the highest sensitivity (62.3), with the T/S ratio (7.3) and background-corrected T/S ratio (30.4) being much lower (p < 0.01). While follow-up scintigraphy aids in assessing thyroid tumor destruction and residual function, its diagnostic utility for differentiating euthyroidism and hypothyroidism is limited, especially for cats with mild (subclinical) hypothyroidism.