Hyperthyroidism is the condition that results from an excess of thyroid hormones. Diagnosis is based on clinical signs, which include hyperactivity and weight loss despite a good appetite, as well as elevated serum thyroid hormone concentrations. Treatment options include radioiodine therapy, antithyroid medication, thyroidectomy, and an iodine-deficient therapeutic diet.
Etiology of Hyperthyroidism in Animals
Clinical hyperthyroidism in cats and dogs results from excessive thyroid hormones, thyroxine (T4) and triiodothyronine (T3).
Functional thyroid adenoma or adenomatous hyperplasia is the most common cause of feline hyperthyroidism; in approximately 70% of cases, both thyroid lobes are affected. Thyroid carcinoma, the primary cause of hyperthyroidism in dogs, is rare in cats, accounting for < 3% of cases.
Iatrogenic hyperthyroidism can occur due to excess levothyroxine administration for the management of canine hypothyroidism, and diet-related hyperthyroidism can occur in dogs fed raw diets containing excessive thyroid gland tissue.
Clinical Findings of Hyperthyroidism in Animals
Hyperthyroidism is most common in cats over the age of 7. The most common clinical signs of hyperthyroidism in animals include the following:
hyperactivity
excessive vocalization
weight loss despite a good appetite
voracious appetite
vomiting
voluminous stool
polyuria-polydipsia
Rarely, hyperthyroid cats exhibit clinical signs of apathy (eg, anorexia, lethargy, and listlessness); weight loss remains a common clinical sign in these cats.
A thyroid nodule is often palpable; however, sometimes the thyroid gland is palpable in euthyroid cats as well (see thyroid nodule image). Tachycardia or tachypnea may be present. The coat may be dry or greasy.
Courtesy of Ontario Veterinary College.
Diagnosis of Hyperthyroidism in Animals
Clinical signs
Serum thyroid hormone testing
High serum total thyroid hormone (T4) concentration is the hallmark of hyperthyroidism and confirms the diagnosis in a clinically affected cat.Serum total T4 concentrations are high in most cats with hyperthyroidism, although approximately 5%–10% of affected cats have normal T4 concentrations, and additional testing is warranted.
In hyperthyroid cats, the CBC is usually normal, although sometimes macrocytosis or microcytosis is present. Increases in ALT and alkaline phosphatase activities are common. Renal function markers should be assessed because they may affect treatment decisions. Urinalysis results are variable; cats with suboptimally concentrated urine (urine specific gravity [USG] < 1.035) are more likely to develop azotemia after treatment.
In a cat with clinical signs of hyperthyroidism, a high T4 concentration is diagnostic for hyperthyroidism, and treatment should be initiated (see hyperthyroidism diagnosis, cats). If a cat has clinical signs of hyperthyroidism and a T4 concentration at the low end of the reference interval, nonthyroidal illness should be investigated as a cause of the signs. With age, T4 concentrations naturally decrease, so a senior or geriatric cat with clinical signs of hyperthyroidism and a T4 concentration at the high end of the reference interval may actually be hyperthyroid.
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In addition, nonthyroidal illness can cause decreases in T4 concentration; if hyperthyroidism is suspected and T4 concentration is in the high end of reference range, additional testing is warranted. A T4 concentration in the high end of the reference interval with an elevated fT4 concentration is diagnostic for hyperthyroidism, and treatment should be started. If T4 and fT4 concentrations are within the reference interval, the cat should be considered to be euthyroid.
Courtesy of Dr. Johanna Heseltine.
Routine screening of T4 concentration in mature cats is common, resulting in the detection of high T4 concentrations in some cats that do not have clinical signs of hyperthyroidism (see routine thyroid testing flowchart). In this situation, the T4 concentration should be rechecked in 2 weeks to exclude a false-positive result, and if it is persistently high, treatment is likely warranted because of the morbidity associated with untreated hyperthyroidism.
If the T4 concentration is at the high end of the reference interval, it should be monitored, or fT4 concentration measurement can be performed to help confirm a diagnosis of hyperthyroidism (by a high-normal T4 concentration with a high fT4 concentration). Free T4 concentration can increase or decrease with nonthyroidal illness, so fT4 concentration is not used as a screening test and should be interpreted with T4 concentration.
Similarly, thyroid-stimulating hormone (TSH) concentration is not sensitive for the diagnosis of hyperthyroidism, and, if testing is performed, TSH concentration should be interpreted with T4 and fT4 concentrations.
If a palpable thyroid nodule is detected without other clinical signs and T4 concentration is normal, rechecking a T4 concentration in 6 months or if clinical signs develop is reasonable.
Courtesy of Dr. Johanna Heseltine.
Complications of hyperthyroidism include thyrotoxic heart disease, hypertension, hypertensive retinopathy, and insulin resistance. Other common comorbidities include chronic kidney disease (which may be masked by hyperthyroidism) and GI disease.
In rare cases, hyperthyroidism is suspected despite T4 and fT4 concentration in the reference interval; a T3 suppression test or thyroid scintigraphy is needed to clarify the patient's thyroid function status.
Treatment of Hyperthyroidism in Animals
Radioiodine therapy
Chronic antithyroid medication
Lifelong iodine-deficient diet
Thyroidectomy
Cats with hyperthyroidism can be treated by radioiodine therapy, chronic administration of an antithyroid drug, lifelong nutritional therapy with an iodine-deficient diet, or thyroidectomy.
Radioiodine therapy is the curative treatment of choice because it is simple and effective. Radioiodine is concentrated within the thyroid gland, where it selectively irradiates and destroys the hyperfunctional thyroid tissue. Radioiodine therapy must be performed at a designated center and requires hospitalization. It avoids anesthesia and the need for ongoing medication administration. Hypothyroidism can result; waiting to administer radioiodine until the TSH is undetectable decreases the risk of this.
Treatment with methimazolecontrols hyperthyroidism by blocking thyroid hormone synthesis. Medication administration is lifelong if used as a sole treatment. The initial dose of methimazole is 1.25–2.5 mg/cat, PO or transdermal, every 12 hours. The dosage is adjusted at 2-week intervals to achieve a T4 concentration in the low- to mid-reference interval. T4 concentration is measured after administration at any time of the day.
Carbimazole is an antithyroid drug similar to methimazole and available in many European countries, Australia, and Japan, and through compounding in the US. It exerts its effects through immediate conversion to methimazole after administration and therefore should not be administered to cats that do not tolerate methimazole.
Propylthiouracil, another antithyroid drug, is not recommended for use in cats because of the high incidence of serious adverse effects (especially hemolytic anemia and thrombocytopenia).
The most common side effect of methimazole is GI upset, which may be mitigated by changing to a transdermal formulation. Serious adverse effects develop in < 5% of treated cats and include agranulocytosis, thrombocytopenia, hepatopathy, or severe facial pruritus. These necessitate discontinuing the methimazole and administering supportive care. These adverse reactions should resolve within 2 weeks.
To achieve a therapeutic dose of methimazole and to monitor for adverse reactions during the first 3 months of treatment (when the most serious adverse effects associated with treatment with methimazole develop), CBC, serum biochemical analysis, and measurement of serum T4 concentration should be repeated at 2-week intervals, with the drug dosage adjusted as needed. Subsequently, serum T4 concentrations should be measured at 3- to 6-month intervals; dosage changes may be required due to progressive hyperplasia of the thyroid gland and to avoid hypothyroidism.
The use of medical treatment other than methimazole (such as iopanoic acid administration) may be required if adverse effects develop from methimazole. For the most part, these alternative medical therapies are for short-term use and are only recommended before a more permanent treatment option.
A prescription diet with severely restricted iodine levels can be used to manage feline hyperthyroidism. The basis is that iodine is an essential component of both T4 and T3, and without sufficient iodine, the thyroid cannot produce excess thyroid hormones. A major indication for use of this diet is a hyperthyroid cat that is not a candidate for radioiodine therapy or thyroidectomy and either cannot be medicated on a daily basis or does not tolerate methimazole.
Most hyperthyroid cats exclusively fed this iodine-restricted diet become euthyroid in 8–12 weeks. This treatment appears to be more effective in cats with only moderate increases of T4 concentration than in cats with severe hyperthyroidism. Cats fed this diet must not eat any other cat food, table food, treats, or prey species because even tiny amounts of iodine can render the diet ineffective at controlling hyperthyroidism. The cat must eat only this diet for the rest of its life.
Surgical thyroidectomy is another potentially curative treatment for hyperthyroidism in cats. Depending on whether the disease affects one or both thyroid glands, surgery may be unilateral or bilateral. If functional ectopic thyroid tissue is present, thyroidectomy will not be effective at treating the hyperthyroidism. After hemithyroidectomy, thyroxine supplementation is usually not necessary. After bilateral thyroidectomy, thyroid function should be assessed 3–4 weeks later to determine whether levothyroxine supplementation is needed.
Thyroidectomy has the associated risk of anesthesia. Hyperthyroid cats should be stabilized before surgery.
Iatrogenic hypoparathyroidism, and subsequently hypocalcemia, may develop after bilateral thyroidectomy and monitoring is needed. If iatrogenic hypoparathyroidism develops, treatment with calcitriol and calcium is indicated. When followed longterm after surgery, > 40% of cats can be persistently or recurrently hyperthyroid, and > 15% are hypothyroid.
In human medicine, percutaneous ethanol injection has been used in treatment of small masses affecting the thyroid and parathyroid glands. In dogs, it has been used for treatment of primary hyperparathyroidism. However, in hyperthyroid cats, it is has been found to be less effective and less safe than other treatment methods and is not recommended.
Propranolol and atenolol are the most frequently used beta-adrenoceptor blocking agents in hyperthyroid cats. These drugs do not lower the circulating T4 concentration but are used to control tachycardia.
For iatrogenic hyperthyroidism in dogs, treatment involves addressing the specific cause (ie, decreasing the levothyroxine dose or changing diets to avoid a raw diet).
Regardless of the treatment method used to treat feline hyperthyroidism, establishing a euthyroid state can unmask renal disease, even in patients with normal renal marker parameters and normal symmetric dimethylarginine concentration prior to treatment. Renal parameters, USG, and blood pressure should be monitored.
Treatment of thyroid neoplasia and hyperthyroidism in dogs is dictated by the size of the primary tumor, extent of local tissue invasion, presence of detectable metastasis, and available treatment options. Surgery, chemotherapy, cobalt irradiation, and use of radioiodine therapy, alone or in combination, may be indicated. The hyperthyroid state can be medically controlled by daily administration of an antithyroid drug (eg, methimazole or carbimazole); however, such treatment will not prevent tumor growth or metastasis.
In dogs, a thyroid tumor causing hyperthyroidism should always be presumed to be a carcinoma until proven otherwise. This is in contrast to the case in cats, in which thyroid carcinoma is present in < 3% of hyperthyroid cats. Because canine hyperthyroidism is almost always associated with thyroid carcinoma, the longterm the prognosis in these dogs is poor to grave.
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Key Points
Hyperthyroidism is a common endocrine disease of mature and senior cats that results from excessive thyroid hormone production. It is uncommon in other species.
Hyperthyroidism causes an increased metabolic rate. Clinical signs often include hyperactivity, excessive vocalization, and weight loss despite a normal or increased appetite.
Diagnosis can be confirmed based on a high T4 concentration. If T4 concentration is in the high end of reference range, a concurrently high fT4 concentration can confirm the diagnosis.
Treatment options include radioiodine therapy, antithyroid medications, a therapeutic prescription low-iodine diet, or thyroidectomy. Each option has advantages and limitations.
For More Information
Bugbee A, Rucinsky R, Cazabon S, et al. AAHA selected endocrinopathies of dogs and cats guidelines. J Am Anim Hosp Assoc. 2023;59(3):113-135. doi:10.5326/JAAHA-MS-7368
Carney HC, Ward CR, Bailey SJ, et al. 2016 AAFP guidelines for the management of feline hyperthyroidism. J Fel Med Surg. 2016;18(5):400-416. doi:10.1177/1098612X16643252
Peterson ME. Hyperthyroidism in cats: considering the impact of treatment modality on quality of life for cats and their owners. Vet Clin Small Anim. 2020;50(5):1065-1084. doi:10.1016/j.cvsm.2020.06.004
Yu L, Lacordia L, Johnstone T. Hyperthyroid cats and their kidneys: a literature review. Aust Vet J. 2022:100(9):415-432. doi:10.1111/avj.13179
Also see pet health content regarding hyperthyroidism in dogs and cats.
