How To Diagnose Hyperthyroidism In Small Animals

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Hyperthyroidism is characterized by hypermetabolism and high circulating thyroid hormone concentrations. Clinical features include weight loss despite a good appetite. Treatment depends on the underlying cause, but options include surgery, radioactive iodine, antithyroid drugs, and low-iodine nutrition.

Clinical hyperthyroidism in cats and dogs is produced from excessive secretion of the thyroid hormones T₄ and T₃, resulting in signs that reverberate an increased metabolic charge per unit. It is most common in middle-aged to old cats but besides develops rarely in dogs.

Functional thyroid adenoma (adenomatous hyperplasia) is the most common cause of feline hyperthyroidism; in ~70% of cases, both thyroid lobes are enlarged. Thyroid carcinoma, the primary crusade of hyperthyroidism in dogs, is rare in cats (1%–2% of hyperthyroidism cases).

The well-nigh common signs of hyperthyroidism in animals include:

weight loss
increased ambition
hyperexcitability
polydipsia
polyuria
palpable enlargement of the thyroid gland

GI signs are likewise common and may include vomiting, diarrhea, and increased fecal volume. Cardiovascular signs include tachycardia, systolic murmurs, dyspnea, cardiomegaly, and congestive middle failure. Rarely, hyperthyroid cats exhibit apathetic signs (eg, anorexia, lethargy, and depression); weight loss remains a common sign in these cats.

High basal serum total thyroid hormone concentration is the hallmark of hyperthyroidism and confirms the diagnosis. Although serum total T₄ concentrations are loftier in most cats with hyperthyroidism, ~v%–10% of cats have normal T_four values. Most cats with normal serum T₄ values take either balmy or early hyperthyroidism or hyperthyroidism with concurrent nonthyroidal illness, which has caused suppression of a high full T_iv concentration to within reference range limits. In these cats, a loftier free T₄ concentration forth with consequent history and physical examination findings is diagnostic of hyperthyroidism.

Radioiodine, thyroidectomy, chronic antithyroid medication, or lifelong iodine-deficient diet

Cats with hyperthyroidism can be treated by radioiodine therapy, thyroidectomy, chronic administration of an antithyroid drug, or lifelong nutritional therapy (iodine-deficient nutrition). Radioactive iodine provides a simple, effective, and safe treatment and is considered the handling of option. The radioiodine is concentrated inside the thyroid tumor, where it selectively irradiates and destroys hyperfunctioning thyroid tissue.

Surgical thyroidectomy is also an effective treatment for hyperthyroidism in cats. With unilateral thyroid tumors, hemithyroidectomy corrects the hyperthyroid land, and thyroxine supplementation usually is non necessary. For bilateral thyroid tumors, complete thyroidectomy is indicated, merely parathyroid function must be preserved to avoid postoperative hypocalcemia. Thyroxine supplementation should exist started 1–2 days after consummate thyroidectomy. If iatrogenic hypoparathyroidism develops, handling with vitamin D and calcium is also indicated.

Handling with methimazole, an antithyroid drug, controls hyperthyroidism by blocking thyroid hormone synthesis. Carbimazole is a similar antithyroid drug available in many European countries, Australia, and Japan; it exerts its effects through immediate conversion to methimazole after administration. Propylthiouracil, another antithyroid drug, is not recommended for utilize in cats because of the loftier incidence of serious agin effects (especially hemolytic anemia and thrombocytopenia). The recommended initial daily dose of methimazole is 2.5–5 mg in two divided doses. The dosage is adjusted to maintain circulating thyroid hormone concentrations inside the mid-normal range and is given daily.

Adverse effects, the more serious of which are agranulocytosis and thrombocytopenia, develop in <5% of treated cats. If this occurs, methimazole should be discontinued and supportive therapy instituted; these agin reactions should resolve within 2 weeks. To maintain normal levels of thyroid hormone and to monitor for agin reactions during the first 3 months of treatment (when the most serious agin effects associated with methimazole therapy develop), CBCs and serum thyroid hormone determinations should be repeated at two- to 4-calendar week intervals, with the drug dosage adapted as necessary. Subsequently, serum T_iv concentrations should be measured at 3- to half dozen-month intervals to monitor dosage requirements and response to treatment.

The use of medical therapy other than methimazole may exist required if adverse effects develop. For the near role, these alternative medical therapies are for short-term utilise and are only recommended before use of a more permanent treatment option.

Propranolol and atenolol are the most frequently used β-adrenoceptor blocking agents in hyperthyroid cats. These drugs practise not lower the circulating T_iv concentration but are used to symptomatically control the tachycardia, tachypnea, hypertension, and hyperexcitability associated with hyperthyroidism.

Oral cholecystographic agents (eg, ipodate, iopanoic acid, or diatrizoate meglumine) acutely inhibit conversion of peripheral T_iv to T₃. In one written report of hyperthyroid cats, administration of calcium ipodate normalized serum total T₃ concentrations and produced clinical improvement in >threescore% of cats treated. Ipodate (308 mg iodine/500 mg calcium ipodate) is no longer marketed in the USA, but iopanoic acid (333 mg iodine/500 mg iopanoic acrid) and diatrizoate meglumine (370 mg iodine/mL) have been used anecdotally in hyperthyroid cats at comparable dosages. None of these drugs provides complete resolution of clinical signs or biochemical features associated with hyperthyroidism. In addition, waning of the thyroid-lowering upshot is common after iii months of therapy with any of these drugs.

A quaternary treatment option for cats with hyperthyroidism is the use of a prescription diet with severely restricted iodine levels (Hill's^® y/d Feline Thyroid Health^TM). The basis for using this diet is that iodine is an essential component of both T_iv and T_iii; without sufficient iodine, the thyroid cannot produce excess thyroid hormones. This is an iodine-deficient nutrition, containing iodine levels below the minimum daily requirement for developed cats. A major indication for use of this diet for direction of feline hyperthyroidism is in cats that are non candidates for definitive treatment of the underlying thyroid tumor(southward) with surgery or radioiodine, which remain the treatments of choice. In addition, nutritional management could be considered in cats whose owners are not able to give oral medication or in cats that develop adverse furnishings from methimazole or carbimazole.

Most hyperthyroid cats exclusively fed this iodine-restricted nutrition become euthyroid in eight–12 weeks. This therapy appears to exist more effective in cats with only moderate increases of T₄ than in cats with astringent hyperthyroidism. Despite some advantages, nutritional management has disadvantages: one) feeding this diet can only control (past withholding "fuel" for the thyroid tumor) just non cure hyperthyroidism; 2) cats fed this diet must not consume whatsoever other true cat diet, table food, or treats, considering even tiny amounts of iodine tin return the diet ineffective in controlling hyperthyroidism; and 3) relapse will occur if the diet is stopped, so the cat must eat just this diet for the rest of its life.

In dogs, a thyroid tumor causing hyperthyroidism should e'er be presumed to be a carcinoma until proved otherwise. This is in contrast to the instance in hyperthyroid cats, in which thyroid carcinoma is nowadays in <v%.

Treatment of thyroid neoplasia and hyperthyroidism in dogs is dictated by the size of the master tumor, extent of local tissue invasion, presence of detectable metastasis, and available treatment options. Surgery, chemotherapy, cobalt irradiation, and employ of radioactive iodine therapy, solitary or in combination, may be indicated depending on the individual. The hyperthyroid state can be medically controlled past daily administration of an antithyroid drug such as methimazole or carbimazole (5–15 mg/dog, twice daily), but such treatment will not forestall tumor growth or metastasis. Considering canine hyperthyroidism is almost e'er associated with thyroid carcinoma, the longterm prognosis in these dogs is poor to grave.

Hyperthyroidism, an overactive thyroid condition, is a common endocrine disorder of older cats. It is uncommon in other species.
Hyperthyroidism causes bodily functions to accelerate. Clinical signs of the disorder include weight loss despite an increased appetite, nervousness, and tachycardia.
Hyperthyroidism is diagnosed based on clinical features, concrete exam findings (particularly palpation of thyroid tumor), and claret tests to mensurate high concentrations of thyroid hormones (total and free T4).
Many treatment options are bachelor, including antithyroid medications that interfere with the production of thyroid hormones (primarily methimazole), surgical thyroidectomy, administration of radioactive iodine, or nutritional management with a depression-iodine diet.