Geriatric endocrinology: Hyperthyroidism (Proceedings)


Geriatric endocrinology: Hyperthyroidism (Proceedings)

Apr 01, 2010

Hyperthyroidism is a common disease of cats over 6 years of age. Feline hyperthyroidism is caused by adenomatous hyperplasia or adenoma of the thyroid gland; carcinomas are rare. The disease is bilateral in 70% of cases. The cause remains unknown. Clinical signs can vary depending upon which stage the disease is identified. Weight loss is present in about 90% of cases with polyphagia in about 50% of cases. Cats can become hyperactivity, anxious, and are prone to this especially when being examined by a veterinarian. Most cases have a thyroid slip.

     Other signs:
          • Polyuria and polydipsia
          • Heart murmur or gallop rhythm
          • Tachycardia
          • Systemic arterial hypertension is common.
          • Vomiting
          • Diarrhea or increased fecal volume
          • Panting or dyspnea
          • Generalized weakness can occur because of impaired muscle function.
          • Cervical ventroflexion is occasionally present and may be due to myopathy, hypokalemia, or thiamine deficiency.

In rare cases apathetic hyperthyroidism can occur with decreased appetite, weight loss and severe lethargy.

Routine laboratory tests

          • Elevated liver enzyme (ALT, AST, alkaline phosphatase) activity is commonly present.
          • BUN and creatinine are elevated in some cats, this is generally related to concurrent renal disease and not the hyperthyroid condition. The presence of this increases risk of treatment for the hyperthyroid state as more severe renal dysfunction may be unmasked.

Diagnostic imaging

     Thoracic radiographs

Indicated when dyspnea, tachypnea, or muffled heart and lung sounds are present.

Ancillary testing

          • Electrocardiography is indicated when an arrhythmia is suspected.
          • Arterial blood pressure should be measured in all cases.

Specific tests for diagnosis

Hyperthyroidism is usually readily diagnosed by documenting elevated serum total T4 concentration. Normal serum T4 can be seen in hyperthyroid cats with mild hyperthyroidism, hyperthyroidism with a concurrent nonthyroidal illness, or a disease other than hyperthyroidism. If the total T4 is normal and hyperthyroidism is still suspected, serum T4 should be measured again in 1-4 weeks. Alternatively free T4 with a dialysis procedure can be run. This is a more sensitive test (98% vs. 90% with just T4). Falsely elevated fT4 can be seen in cats with nonthyroidal illness.

     Thyrotropin releasing hormone (TRH) stimulation test

As effective as the T3 suppression test in diagnosing hyperthyroidism in difficult cases.

          • Obtain blood sample for serum T4 concentration before and 4 hours after IV administration of 0.1 μg/kg TRH.

          • Normal response is an increase in T4 on the 4 hour sample > 60% of the basal concentration.
          • Hyperthyroidism is diagnosed when the serum T4 concentration is < 50% of baseline.

     T3 suppression test

          • Obtain blood sample prior to initiating test for measurement of serum T4.
          • Administer T3 at 25 μg PO q 8 hours for 7 doses.
          • Obtain blood sample 2-4 hours after the final dose of T3 for measurement of serum T4 and T3 concentrations.

          • Normal response is a decrease in T4 concentration to < 20 nmol/L.
          • Hyperthyroidism is diagnosed when the serum T4 is > 20 nmol/L post-T3 administration.
          • Serum T3 should be elevated on the post-treatment sample unless the T3 was not administered properly.