Lisa Stammeleer 1,2,*, Pilar Xifra 3, Sara I Serrano 3, Eva Vandermeulen 4,5, Sylvie Daminet 1, Mark E Peterson 6,7,*
Background
Thyroid scintigraphy is considered the gold standard for diagnosing and staging hyperthyroidism in cats and determining appropriate radioiodine (¹³¹I) dosing. While post-treatment outcomes are commonly assessed via serum thyroid hormone concentrations, the role of follow-up scintigraphy in evaluating treatment success and residual thyroid function remains poorly characterized. This study aims to describe both qualitative and quantitative scintigraphy findings in a large cohort of hyperthyroid cats before and six months after ¹³¹I therapy and to evaluate their correlation with thyroid function.
Methods
This was a multicentric, prospective before–after study involving 234 hyperthyroid cats treated with ¹³¹I at three centers. Thyroid scintigraphy using ⁹⁹ᵐTc-pertechnetate was conducted before and six months after treatment. Cats were classified post-treatment as euthyroid, subclinically hypothyroid, or overtly hypothyroid based on serum T4 and TSH levels. Quantitative measures included thyroid-to-salivary gland ratio (T/S), background-corrected T/S (cT/S), and percent thyroidal uptake (TcTU). Statistical analyses assessed diagnostic performance and group differences.
Results
Of the 234 cats, 165 (70.5%) became euthyroid, 54 (23.1%) were subclinically hypothyroid, and 15 (6.4%) developed overt hypothyroidism post-treatment. All cats showed decreased nodule size and radionuclide uptake. In unilateral disease cases, 61% had contralateral thyroid lobe function recovery. Persistent “hot” nodules were found in 26 cats (11%) who remained euthyroid. Conversely, 24 cats (10.3%) had no visible thyroid tissue post-treatment, with 71% being hypothyroid. TcTU showed higher sensitivity (62.3%) for diagnosing hypothyroidism compared to T/S (7.3%) and cT/S (30.4). Specificity for all parameters remained high.
Limitations
The study's limitations include lack of long-term follow-up in all cats and the exclusive use of ⁹⁹ᵐTc-pertechnetate instead of ¹²³I, which may have provided better insights into hormone synthesis and residual tissue function. Additionally, recovery of suppressed thyroid tissue may have occurred beyond the six-month follow-up window.
Conclusions
While follow-up scintigraphy confirms destruction or reduction of thyroid tumor tissue and can illustrate functional recovery of suppressed thyroid lobes, it has limited diagnostic value in distinguishing euthyroid from mildly hypothyroid cats. TcTU is the most reliable quantitative indicator but still has modest sensitivity. Therefore, serum T4 and TSH measurements remain essential for evaluating post-treatment thyroid status.
Thyroid scintigraphy in four cats with unilateral thyroid nodules, before and after treatment with 131I. (A) Reduction in size and radionuclide uptake of the “hot” right-sided thyroid nodule, with recovery of the previously suppressed left thyroid lobe; (B) Complete ablation of unilateral left thyroid nodule, with recovery of the contralateral right lobe; (C) Reduction in size and uptake intensity of the left thyroid nodule, but the contralateral right lobe remains suppressed. (D) Complete ablation of both the hot nodule and normal contralateral lobe
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