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Late stage hyperthyroidism in cats

Late stage hyperthyroidism in cats


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Late stage hyperthyroidism in cats is frequently refractory to medical therapy, with recurrent hyperthyroidism after antithyroid drug administration. Surgical therapy is indicated in cats with severe hyperthyroidism. This article reports clinical findings, diagnostic approaches, and outcomes after a median sternotomy for a thyroideal fistula repr in two cats.

The case was a 4‐year‐old female castrated Domestic Shorthr cat with a history of recurrent hyperthyroidism that had been treated with carbimazole for the past 1.5 years. The owner reported that the cat’s hyperthyroidism had been gradually progressing and that the cat was exhibiting progressive signs of hyperthyroidism for the past 3 months. Physical examination results were consistent with the diagnosis of hyperthyroidism, including tachycardia (heart rate, 280 beats/min), heat intolerance, polyphagia, increased body weight (12.8 kg), polyuria, polydipsia, and a diffuse goiter.

Serum biochemical profile revealed high concentrations of free T4 (8.8 ng/dL, reference range [RR], 2.2 to 5.8 ng/dL), free T3 (9.9 pg/mL, RR, 2.0 to 3.6 pg/mL), and thyroid‐stimulating hormone (TSH) (2.2 μIU/mL, RR, 0.2 to 1.6 μIU/mL). Thyrotropin receptor antibody was negative.

Thyroid scintigraphy was performed on two occasions, 2 days apart. The cat received 3.4 mCi (125)I (99.9% ^131^I) within 2 days of the first scintigraphy, after 24 hours, a subsequent scintigraphy revealed a large area of increased radiotracer concentration in the neck consistent with a thyroideal fistula ([Figure 1](#f1){ref-type="fig"}). A median sternotomy was performed to explore the possible surgical repr.

![(A) Thyroid scintigraphy at 24 hours. (B) Thyroid scintigraphy at 2 weeks. The thyroid scintigraphy images show an area of increased radiotracer concentration at the thyroideal fistula in the caudal cervical neck (arrow) at 24 hours after the (125)I (99.9%) injection, and at 2 weeks postoperatively the increased radiotracer concentration was significantly reduced at the surgical site (arrow) (A).](JVIM-30-835-g001){#f1}

At surgery, a 5‐mm skin incision was made at the level of the sternal body, and the sternal body was opened along the midline with a sharp dissection. The sternal body was elevated, and the pericardium was cut to expose the mediastinum. The mediastinum was opened along the midline and the trachea was visualized. A 1.5‐cm longitudinal skin incision was made along the midline, and the skin was elevated to reveal the thyroid and recurrent laryngeal nerves. An enlarged and friable thyroid was present at the level of the caudal cervical thyroid lobe. The thyroid was excised, and a large thyroideal fistula was visualized. A large area of scar tissue was present at the level of the esophagus, with mild fibrosis in the surrounding mediastinal tissue, and a fibrotic band was noted between the thyroid and esophagus. The fistula was closed using a running suture, with two separate interrupted absorbable 4‐0 monofilament sutures. The skin incision was closed using two separate running 3‐0 monofilament absorbable sutures. The pericardium was closed using two running 3‐0 monofilament absorbable sutures. The wound was closed in layers.

The cat was monitored dly after surgery. Postoperatively, the cat developed fever (102°F), tachycardia (heart rate, 290 beats/min), and mild tachypnea. The cat was hospitalized for 3 days. The cat's hyperthyroidism was resolved within 7 days after surgery. The cat was then started on a dly oral mntenance dose of 0.7 mg/kg of carbimazole.

The second case was a 5‐year‐old castrated Domestic Shorthr cat with a history of hyperthyroidism that had been treated with oral methimazole for 1 year. The owner reported that the cat's hyperthyroidism had been gradually progressing and that the cat was exhibiting progressive signs of hyperthyroidism for the past 3 months. Physical examination results were consistent with the diagnosis of hyperthyroidism, including tachycardia (heart rate, 270 beats/min), increased body weight (16.5 kg), polyphagia, polyuria, and polydipsia.

In this case, the cat underwent a thyroideal fistula repr. Serum biochemical profile revealed hyperthyroidism (free T4, 5.6 ng/dL, RR, 2.3 to 4.5 ng/dL, free T3, 14.1 pg/mL, RR, 2.2 to 3.4 pg/mL, and TSH, 3.1 μIU/mL, RR, 0.2 to 1.6 μIU/mL). Thyrotrop


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