Levothyroxine Sodium Powder

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lee-vo-theye-ROKS-een SO-de-um POW-der - Pronunciation guide

Brand Names

  • Equi-Phar Thyrosyn Powder
  • Equine Thyroid Supplement
  • Thyroxine-L Powder


Rx symbolLevothyroxine sodium powder is a synthetically produced drug that is similar to the primary secretion of the horse's thyroid gland. It occurs as an odorless, light yellow to buff-colored, tasteless, hygroscopic powder that is slightly soluble in both water and alcohol. It can be injected, or the powder form may be mixed with corn syrup or molasses. The prescribed dose is fed in this form or used as a top dressing on grain.


Levothyroxine sodium powder is used in horses for correction of a low-circulating thyroid hormone condition known as hypothyroidism.

Dosage and Administration

Prescription medicationLevothyroxine Sodium Powder
Method Dosage
(click row for calculator)
Concentration Period Duration
Oral1 0.5-3 mg/100lb 2.156 mg/g Once or twice daily NA


  • Federal law restricts this drug to use by or on the order of a licensed veterinarian.
  • Extra-label use of drugs in treating animals is allowable only by licensed veterinarians within the context of a valid veterinarian-client-patient relationship, and does not include drug use in treating animals by the layman (except under the supervision of a licensed veterinarian).
  • 1Thyroxine-L Powder can be administered by mixing the daily dose in the concentrate or by top dressing on the grain, preferably rolled or ground. To facilitate the proper adhesion of Thyroxine-L Powder to the ration, slightly moisten the grain with water or a liquid supplement.
  • Dosages should be individualized and animals should be monitored daily for clinical signs of hyperthyroidism or hypersensitivity.
  • Calculator is for educational purposes only. Follow your veterinarian's instructions regarding use of this, or any medication.

Side Effects

When administered at an appropriate dose, no adverse effects have been reported.


Bioavailability differences have been reported in different products. It is recommended that a reputable brand be selected, and that brands should not be indiscriminately chosen or changed.

Compliance with therapy as prescribed is very important and the horse should be monitored closely for symptoms of hyperthyroidism, including tachycardia, polyphagia, excitability, nervousness, and excessive panting.

Levothyroxine should not be used in horses with myocardial infarction, thyrotoxicosis, or untreated adrenal insufficiency. It should be used with caution in horses that have cardiac disease, diabetes, or are advanced in age.

Levothyroxine sodium is FDA-approved for use in horses. It is a prescription drug restricted to use by or on the lawful written or oral order of a licensed veterinarian.

In cases of competition horses, check with the proper individual regulatory body regarding use.


Levothyroxine increases the actions of epinephrine, norepinephrine, and other catecholamines and sympathomimetics. It may increase the anti-coagulation effects in horses on warfarin. In horses with diabetes, thyroid hormones may alter insulin requirements. Levothyroxine may decrease therapeutic effects of digoxin or digitoxin.


Chronic and/or acute overdosage will produce symptoms of hyperthyroidism, excitability, nervousness, and excessive panting. If this occurs, a veterinarian should be called. Treatment is supportive and symptomatic. Oxygen, artificial ventilation, cardiac glycosides, beta blockers, fluids, and antipyrexic agents may be necessary in extreme cases.


PalaTech Levothyroxine Sodium PowderPalaTech Levothyroxine Sodium Powder

Thyro-L Levothyroxine Sodium Powder Thyro-L Levothyroxine Sodium Powder




About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.