Feline Hyperthyroidism

• Clinical signs of hypertyroidism

  Increased appetite

  Vomiting

  Hyperactivity

  Weight loss

  PU/PD

  Muscle wasting

  Cardiac murmur
• Thyroid storm or crisis?

  • Rare but life-threatening complication of hyperthyroidism described in people.
  • Exaggerated manifestation of thyrotoxicosis (hyperthermia, CNS signs, cardiovascular abnormalities).
  • In cats; Thyroid crisis = severe tachycardia (>300bpm), tachypnoea, panting, respiratory distress, profound weakness, ventroflexion, sudden blindness.
• Specific tests for Hyperthyroidism

  • Total T4 - increased in 90-95% of hyperthyroid cats, day to day fluctuations, suppresses by non-thyroidal diseases, immunoassays may underestimate the level.
  • Free T4 - unbound, can diffuse into the cell, more sensitive but increases chances of false positive.
  • T3 - active form of thyroid hormone, derived from T4 in extrathyroidal tissues.
  • TSH - canine, non-thyroid al illness affects it.
• Radioactive iodine for Hyperthyroidism

  Administration of radioactive iodine via a subcutaneous injection.

  Simple and effective but requires licensing and hospitalisation facilities.

  Cats become radioactive - have to be isolated.

  Curative.

  Targets the euthyroid not the hypothyroid.
• Surgical therapy for hyperthyroidism

  65% of cats have bilateral disease.

  These tumours can be almost anywhere in the neck, can identify them with an ultrasound.

  Do not remove the parathyroid gland, if you remove both thyroids and both parathyroids you will induce hypothyroidism.
• Medical therapy for hyperthyroidism

  Anti-thyroid drugs (carbimazole, methimazole) - oral, transdermal ointment, liquid. This makes administration to cats a lot easier but the medications have to be used all the time once started.

  There is always a risk that the adenomas will become malignant in the future.
• Adverse reactions of medical therapy for hyperthyroidism

  Non-life threatening adverse reactions:

  Anorexia, vomiting, lethargy.

  Face and neck excoriation.

  Life-threatening adverse reactions:

  Dyscrasia (leukaemia, anaemia, thrombocytopenia)

  Hepatopathy.
• Dietary management of hyperthyroidism

  Ultra-low iodine diet - it manages but does not cure the disease or reverse the adenomatous changes in the thyroid.

  General nutritional assessment

  Weight loss - mainly muscle loss

  Sarcopenia of aging
• Monitoring of the hyperthyroid cat

  Fund dependant

  Radioactive iodine - unmasking kidney disease and becoming azotaemic, potential for iatrogenic hypothyroidism. TSH/T4

  Surgery - parathyroid related problems (if damaged), if unilateral monitor for reoccurrence. T4

  Anti-thyroid medication - T4, biochemistry/haemotology

  Low iodine diet - T4 only.