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Liothyronine (Cytobin®)

By: Dr. Karin Szust

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  • Thyroid hormones are produced by two glands located in the neck. These hormones support normal cell function in almost every organ of the body.
  • The main thyroid hormone in dogs and cats is levothyroxine (also called L-thyroxin or T4). L-thyroxin is later converted to another more active thyroid hormone called T3 or triiodothyronine. If this drug is not effective, liothyronine is often prescribed as an alternative.
  • The thyroid hormones have significant beneficial effects on brain, heart, muscle, nerve and kidney functions and also support a healthy skin and hair coat.
  • When an individual cannot manufacture enough thyroid hormone to support normal cell metabolism, a condition called hypothyroidism develops.
  • In dogs, an autoimmune disease directed against the thyroid glands usually causes hypothyroidism, and this is often suspected in a dog with lethargy, poor hair coat, obesity and weakness.
  • In cats, hypothyroidism is relatively rare, and usually occurs when the thyroid glands are destroyed following thyroid gland surgery or after treatment for an over-active thyroid gland (a common condition in cats called hyperthyroidism – the opposite of hypothyroidism).
  • A veterinarian can only make a diagnosis of hypothyroidism after analysis of special blood tests.
  • It should be noted that some sick pets have slightly low thyroid hormone levels in the blood; however, this does not necessarily mean the pet has hypothyroidism. Other illnesses can temporarily depress thyroid hormone concentration.
  • Liothyronine is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.

    Brand Names and Other Names

  • This drug is registered for use in humans and animals.
  • Human formulations: Cytomel® (SK Beecham) and various generic preparations
  • Veterinary formulations: Cytobin® (Pfizer)

    Uses of Liothyronine

  • Liothyronine is used to treat hypothyroidism in patients that do not respond to levothyroxine supplementation.

    Precautions and Side Effects

  • While generally safe and effective when prescribed by a veterinarian, liothyronine can cause side effects in some animals.
  • Liothyronine should not be used in animals with known hypersensitivity or allergy to the drug.
  • Liothyronine may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with liothyronine.
  • Thyroid hormone supplementation should never be administered to animals without a definite diagnosis of hypothyroidism.
  • Excessively high doses of liothyronine can accumulate and lead to excitement, tremors and excessive stimulation of the heart. This problem may easily be overlooked.
  • To ensure effective and safe dosing of liothyronine, blood tests that measure thyroid hormone levels must be obtained once the pet is receiving a consistent dose of thyroid hormone supplement.

    How Liothyronine Is Supplied

  • Cytobin tablets are available in 60 mcg (micrograms) and 120 mcg.
  • Cytomel tablets are available in 5 mcg, 25 mcg and 50 mcg.

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • The typical dose administered to dogs is 2 to 3 mg per pound (4 to 6 mcg/kg) orally every 8 hours, but some dogs require less than this.
  • The dose for cats is 2 mg per pound (4.4 mcg/kg) two to three times a day orally.
  • Final dose adjustments are made on the basis of blood tests to measure thyroid hormone levels.

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