Hyperthryoidism

Hyperthyroidism is common in older cats and is usually caused by thyroid gland hyperplasia or benign thyroid adenoma(s). Many affected cats eventually experience weight loss, polyphagia, polydipsia and polyuria, gastrointestinal abnormalities, and restlessness. Tachycardia and cardiac arrhythmias may be detected and secondary cardiomyopathy is common. Other clinical findings include a palpable cervical mass, thin/unkempt appearance, dehydration, and weakness. Because hyperthyroid cats are often geriatric, concurrent illness(es) may be detected.

The most consistent findings on routine bloodwork are mild to moderate increases in ALT and ALP activities. The increased ALP activity is from both liver and bone isoforms. Azotemia may be detected and USG will help distinguish prerenal from renal causes. Other hematologic and biochemical findings are variable and relate, in large part, to whether concurrent disease is present. Erythrocytosis may be present in 40-50% of hyperthyroid cats, particularly those without concurrent illness; the mechanism is likely related to a direct effect of thyroid hormone on bone marrow erythroid precursors and its stimulation of erythropoietin production.

Total serum T4 is elevated in about 90% of hyperthyroid cats. Total serum T3 is less consistently elevated. When the clinical suspicion of hyperthyroidism is high despite a total serum T4 within the RI, serum free T4 can be measured. Alternatively, the total T4 can be rechecked at a later time (total T4 exhibits daily fluctuation), or a T3 suppression test can be done. The T3 suppression test (see protocol manual) is based on the fact that exogenous T3 given orally to normal individuals over 2 days will suppress TSH release from the pituitary and consequently, T4 release will be decreased. With hyperthyroidism, TSH secretion is already suppressed and exogenous T3 will have no additional effect on T4. Serum T3 is measured only to ensure that the cat received the exogenous T3. Measurement of T4 in a very ill cat is generally not recommended, unless the illness is thought to be due to hyperthyroidism itself (e.g. thyrotoxic crisis, which is uncommon). If clinical illness is thought to be nonthyroidal in origin (e.g. signs due to renal failure), it is preferable to stabilize the patient and evaluate thyroid function when the cat is feeling better. This will help avoid testing at a time-point when the T4-lowering effects of nonthyroidal illness could lead to a misdiagnosis.

Medical antithyroid treatment of hyperthyroidism necessitates monitoring of the total serum T4 since iatrogenic hypothyroidism can occur and dosage adjustment may be required. Radioactive iodine treatment also requires long-term monitoring for incomplete response to treatment or hypothyroidism. Both total serum T4 and serum calcium must be monitored following surgical treatment of hyperthyroidism since the parathyroid glands may be inadvertently removed with the thyroid glands, resulting in hypocalcemia. Hyperthyroidism increases GFR which is beneficial to the kidneys in cats with concurrent renal disease, however with effective treatment of hyperthyroidism renal function may be further compromised. Therefore the benefits of treating hyperthyroidism versus the possibility of exacerbating the renal disease must be weighed and hyperthyroid cats with concurrent renal disease should have their renal function closely monitored once treatment for hyperthyroidism begins.

Hyperthyroidism is rare in domestic species other than the cat. Thyroid tumors occur in dogs and most of these are aggressive, invasive follicular carcinomas. Generally thyroid carcinomas are nonfunctional and affected dogs are euthyroid (normal thyroid function) or hypothyroid from destruction of normal thyroid tissue. Rarely, thyroid carcinomas are functional and dogs are presented with signs of hyperthyroidism in addition to the presence of a cervical mass. Thyroid adenomas may occur in adult horses, but are not associated with hyperthyroidism.