Lesson 10, Topic 4
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Therapeutic Goals:

1. Control the cardiac manifestation of hyperthyroidism which occurs a result of excessive sympathetic stimulation.

  1. If the echocardiographic picture is one of concentric hypertrophy, treat as per feline hypertrophic cardiomyopathy
  2. If the echocardiographic picture is one of eccentric hypertrophy, treat as per feline dilated cardiomyopathy
  3. If dysrhythmias are present (premature beats):
    • Beta blockers tend to be the general class of antidysrhythmics of choice. They may also be effective if sinus tachycardia is present.
      • Propranolol: 0.2 – 1 mg/kg BID-TID (PO)
      • Metoprolol: 5-15 mg TID (PO)
      • Atenolol: 5-12.5mg SID-BID (PO)
      • Propranolol has drawbacks in its combined beta 1 and beta 2 blocking activity (promotes bronchoconstriction, this is particularly worrisome if pulmonary edema is present). The selective beta blockers metoprolol and atenolol are preferable in patients with respiratory distress.
  4. Antithyroid medication – to reduce the systemic effects of hyperthyroidism:
    • Methimazole – 5 mg BID-TID (PO)
  5. Surgical excision of thyroid tumors results in long term cure. Surgical procedures must attempt to preserve at least 1 parathyroid gland. Schedule surgery when cat is euthyroid.
  6. Radioactive Iodine:
  • Simple, effective, and safe
  • Radioactive iodine is concentrated primarily in the hyperplastic or neoplastic thyroid cells irradiating and destroying the hyperfunctioning tissue.
  • Normal thyroid tissue is protected from the effects of radio active iodine since the uninvolved thyroid tissue is suppressed and receives only a small dose of radiation
  • Requires a nuclear facility

Comments: Adverse drug reactions

  1. General signs:
    • Anorexia, vomiting, and lethargy. These signs are transient and usually resolve despite continued drug therapy
    • Adverse GI signs may persist –> stop drug
  2. Increased renal parameters reflecting reduced GFR – subclinical renal disease may be unmasked.
  3. Granulocytopenia may occur with methimazole. CBCs must be monitored.
  4. If adverse effects occur, recommend treat with beta blocker alone (use with caution if feline DCM or intermediate CM is present).

Comments: Site of tumour

Because the thyroid lobes of the cat are loosely attached to the trachea, the enlarged lobe(s) frequently descend ventrally from its normal location adjacent to the larynx. Many cats in which a thyroid mass is not palpable have tumorous lobes that have descended into the thoracic cavity. 70% of cases have bilateral thyroid lobe involvement. 30% of cases have single thyroid lobe involvement. Of those cases with true unilateral lobe involvement, recurrence in the contralateral lobe is unusual. Of those cases with bilateral lobe involvement, about 15% of these have one lobe that is minimally enlarged and is usually mistaken as normal. Therefore if only one lobe is removed, relapse of hyperthyroidism will usually occur within 9 months of surgery. Preservation of the external parathyroid gland during hemithyroidectomy minimizes the risk of hypoparathyroidism should removal of the contralateral lobe be required. It can be difficult to remove all abnormal thyroid tissue while attempting to concurrently preserve parathyroid tissue. Small remnants of thyroid tissue that remain attached to the parathyroid may regenerate and produce thyrotoxicosis in 6 to 12 months. Hypocalcemia is the most important complication associated with bilateral thyroidectomy. It doesn’t occur with unilateral thyroidectomy. After bilateral thyroidectomy, the serum calcium concentration should be monitored on a daily basis until it has stabilized within the normal range. After bilateral thyroidectomy hypocalcemia will develop in 1 to 3 days if it is to occur. Although hypocalcemia may be permanent in some cats, spontaneous recovery of parathyroid function may occur weeks to months after surgery. In most cases, such transient hypocalcemia results from reversible parathyroid damage and ischemia secondary to surgery. Alternatively, accessory parathyroid tissue may compensate for the damaged parathyroid glands and maintain normocalcemia.