A more recent paper described the use of ethanol ablation in four cats.¹¹ The cats had increased serum T₄ concentrations, a palpable thyroid nodule, and clinical signs of hyperthyroidism. Unilateral disease was identified with nuclear scintigraphy, and an enlarged single thyroid mass was found in each cat by using ultrasonography. The thyroid mass volumes were 0.20, 0.33, 0.74, and 0.84 cm³. The cats received intravenous propofol to prevent movement during the 10-minute injection procedure. Ultrasound guidance was used to place a 27-ga needle into the thyroid mass. Ethanol (96%) was slowly injected until it diffused throughout the mass or an injection volume of half the calculated mass volume was given. All four cats received a single injection, and within 48 hours serum T₄ and free T₄ concentrations became normal or subnormal. These hormone concentrations remained below or within the reference range during the six-month study. The owners reported that their cats' clinical signs of hyperthyroidism resolved within one week of the treatment. The cats did not exhibit clinical signs of hypothyroidism or require thyroid supplementation. Nuclear scintigraphy done three months after the injection procedure revealed normal results in one cat and faint residual increased pertechnetate uptake in three cats. Severe serum chemistry profile abnormalities or clinical complications were not seen after the ethanol injection or during the study. The owners noticed a transient voice change in two of the cats within a few days of the injection, but it resolved six to eight weeks later.

Heat ablation

A recent study evaluated ultrasound-guided heat ablation of thyroid tissue in nine cats with hyperthyroidism.¹² While treatment was successful in lowering total T₄ concentrations in all cats, the mean duration of euthyroidism was only four months, and hyperthyroidism recurred in all cats within 18 months after treatment.

Many cats successfully treated for hyperthyroidism experience a decrease in renal function, though only a small percentage experience severe azotemia or clinical signs. The first study to evaluate the effects of treating hyperthyroidism on renal function involved 58 cats.¹³ Urine specific gravity and serum creatinine, blood urine nitrogen (BUN), and T₄ concentrations were measured before and 30 and 90 days after the cats' hyperthyroidism was treated with radioactive iodine, methimazole, or bilateral thyroidectomy. Thirty and 90 days after treatment, the mean serum creatinine and BUN concentrations were significantly higher than pretreatment concentrations. The mean serum creatinine, BUN, and T₄ concentrations did not differ among groups before treatment or 30 and 90 days after treatment. This finding was important as it showed that the treatment itself was not the problem; instead, the worsening azotemia may have resulted from reversal of the hyperthyroid and hypermetabolic state. Reducing serum T₄ concentrations by treating hyperthyroidism may cause azotemia in older cats with chronic renal disease. The authors concluded that it may be prudent to treat azotemic (or, in fact, all) hyperthyroid cats with methimazole until you can determine whether correcting the hyperthyroid state will cause a clinically relevant reduction in renal function.