Side Effects of Levothyroxine Sodium in Horses

Levothyroxine sodium is an equine synthetic thyroid hormone used to correct conditions associated with hypothyroidism. Thyroid hormone regulates metabolic rate and blood calcium levels. Horses with hypothyroidism may have stunted growth and abnormalities of the central nervous system.

  1. Significance

    • There are no known side effects if correct dosages are given, according to Doctors Foster and Smith Pharmacy. Allergic reaction side effects include hives, scratching, swelling of the face, whitish gums, diarrhea, seizures, shock, cold limbs or coma. Levothyroxine sodium increases the drug effects of warfarin, epinephrine and norepinephrine and decreases the effect of digoxin. Diabetic horses may require increased dosages of insulin.

    Considerations

    • Caution is urged when using levothyroxine sodium to treat older horses and those with heart disease, high blood pressure or Addison's disease. It should not be given to horses with hyperthyroidism, a condition that causes the body to produce a large amount of thyroid hormone. There has been no determination of the safety of this medication in mares that are pregnant or nursing.

    Warning

    • Follow dosage recommendations as prescribed by your veterinarian. To adjust the dose, it may be necessary to monitor the thyroid hormone levels in the blood every two to four weeks. Horses that have consistently taken a slightly higher dose of levothyroxine may show signs of hyperthyroidism such as excessive drinking, eating and urinating. Other signs include weight loss, raised heart rate, agitation and behavioral changes.