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Levothyroxine

PrescriptionCDSCO approved
Synthetic thyroid hormone (T4)
Last reviewed 19 Apr 2026 · PetCare.AI Editorial Team
Species
Dog, Cat
Brands
4 available
Interactions
4 documented
Formulations
5

Mechanism of action

Synthetic L-thyroxine (T4) that is peripherally converted to triiodothyronine (T3), restoring normal thyroid hormone-mediated metabolic processes in hypothyroid patients

At a glance

Class
Synthetic thyroid hormone (T4)
Schedule
Prescription
Storage
Store below 25°C, protect from light and moisture
CDSCO (India)
Vet-approved — 1970-01

Dosing

🐕

Dog

Hypothyroidism
Dose
0.02–0.04 mg/kg
Route
PO
Frequency
BID (may reduce to SID once stable)
Max dose
0.8 mg/dose; 1.6 mg/day
Duration: Life-long; adjust based on T4 levels
🐈

Cat

Hypothyroidism (iatrogenic, post-thyroidectomy or radioiodine)
Dose
0.05–0.2 mg/cat total
Route
PO
Frequency
SID to BID
Max dose
0.2 mg/dose; 0.4 mg/day
Duration: Life-long if persistent

Formulations

💊

Tablet — 5

Strength
0.1mg
Available in India
Strength
0.2mg
Available in India
Strength
0.3mg
Available in India
Strength
0.5mg
Available in India
Strength
0.8mg
Available in India

Storage

Store below 25°C, protect from light and moisture

Safety

Absolute contraindications — do not use

  • Uncorrected adrenal insufficiency
    Treat Addison's first; thyroid replacement increases cortisol demand

Use with caution

  • Cardiac disease
    Start at low dose and titrate slowly
  • Diabetes mellitus
    May alter insulin requirements

Adverse effects

Common
Generally well tolerated at correct dose
Serious
Thyrotoxicosis (overdose: tachycardia, polyuria, polydipsia, weight loss, restlessness)
Cardiac arrhythmias

Monitoring parameters

Total T4 (4–6h post-pill)Free T4 by equilibrium dialysisTSHClinical signs (coat quality, weight, energy)Recheck 4–8 weeks after initiation/dose change

Interactions

Major — 1

Cholestyramine
major
Cholestyramine binds levothyroxine in the GI tract, reducing absorption by 50-90%. Renders thyroid replacement therapy ineffective.
Management: Separate by at least 4-6 hours. Give levothyroxine on empty stomach, cholestyramine with meals. Monitor T4.

Moderate — 3

Phenobarbital
moderate
Phenobarbital increases hepatic metabolism of thyroid hormones, potentially causing falsely low T4 levels
Management: Monitor T4 levels. May need to increase levothyroxine dose. Free T4 by equilibrium dialysis is more reliable.
Sucralfate
moderate
Sucralfate binds thyroid hormone in the gut, reducing levothyroxine absorption
Management: Give levothyroxine at least 4 hours before sucralfate.
Calcium Gluconate
moderate
Calcium chelates levothyroxine in the GI tract, reducing absorption by up to 40%.
Management: Separate by 4 hours. Give levothyroxine on empty stomach, calcium supplements with meals.

Brands

International

Thyro-Tabs
Lloyd/Dechra
Soloxine
Virbac

India

Eltroxin
Glaxo
Thyronorm
Abbott India

FAQs

Frequently asked questions

›What is Levothyroxine?
Levothyroxine is a synthetic thyroid hormone (t4) used in pets. Synthetic L-thyroxine (T4) that is peripherally converted to triiodothyronine (T3), restoring normal thyroid hormone-mediated metabolic processes in hypothyroid patients
›What is Levothyroxine used for in pets?
Levothyroxine is used in veterinary medicine for: Hypothyroidism; Hypothyroidism (iatrogenic, post-thyroidectomy or radioiodine).
›What is the Levothyroxine dose for dogs?
For dogs, Levothyroxine is typically dosed as follows — Hypothyroidism: 0.02–0.04 mg/kg PO BID (may reduce to SID once stable). Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.
›What is the Levothyroxine dose for cats?
For cats, Levothyroxine is typically dosed as follows — Hypothyroidism (iatrogenic, post-thyroidectomy or radioiodine): 0.05–0.2 mg/cat total PO SID to BID. Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.
›What are the side effects of Levothyroxine?
Common: Generally well tolerated at correct dose. Serious (call your vet immediately): Thyrotoxicosis (overdose: tachycardia, polyuria, polydipsia, weight loss, restlessness), Cardiac arrhythmias.
›Does Levothyroxine need a prescription?
Yes. Levothyroxine is a prescription medication and should only be administered under veterinary supervision.
›When should Levothyroxine not be used?
Do not use Levothyroxine if: Uncorrected adrenal insufficiency.

References

References

Textbooks & handbooks

  • Plumb, D.C. Plumb's Veterinary Drug Handbook. 10th ed., Wiley-Blackwell, 2023.
  • Vail, D.M., Thamm, D.H., & Liptak, J.M. (eds.). Withrow & MacEwen's Small Animal Clinical Oncology. 6th ed., Saunders/Elsevier, 2020.
  • Riviere, J.E., & Papich, M.G. (eds.). Veterinary Pharmacology and Therapeutics. 10th ed., Wiley-Blackwell, 2018.
  • National Research Council. Nutrient Requirements of Dogs and Cats. National Academies Press, Washington DC, 2006.
  • The Merck Veterinary Manual. Merck & Co., Online edition. https://www.merckvetmanual.com/

Clinical guidelines & consensus

  • Fletcher, D.J., Boller, M., Brainard, B.M., et al. "RECOVER Evidence and Knowledge Gap Analysis on Veterinary CPR." Journal of Veterinary Emergency and Critical Care, 2012;22(S1):S102–S131.
  • American Animal Hospital Association. 2018 AAHA Diabetes Management Guidelines for Dogs and Cats. AAHA Press.

Journals & peer-reviewed studies

  • Hogan, D.F., Fox, P.R., Jacob, K., et al. "Secondary prevention of cardiogenic arterial thromboembolism in the cat: The FAT CAT study." Journal of Veterinary Cardiology, 2015;17(Suppl 1):S306–S317.
  • Boswood, A., Häggström, J., Gordon, S.G., et al. "Effect of Pimobendan in Dogs with Preclinical Myxomatous Mitral Valve Disease and Cardiomegaly: The EPIC Study — A Randomized Clinical Trial." Journal of Veterinary Internal Medicine, 2016;30(6):1765–1779.
  • ASPCA Animal Poison Control Center. Toxicology and Poison Management Guidelines. American Society for the Prevention of Cruelty to Animals. https://www.aspca.org/pet-care/animal-poison-control

Regulatory & approvals

  • Central Drugs Standard Control Organisation (CDSCO), Government of India. Veterinary Drug Approval Registry, 1969–2026. Directorate General of Health Services. https://cdsco.gov.in/

Databases

  • Washington State University, College of Veterinary Medicine. Veterinary Clinical Pharmacology Laboratory (VCPL) — MDR1 Multidrug Sensitivity Database. https://vcpl.vetmed.wsu.edu/
Educational reference only
This information is provided for educational purposes and is not a substitute for professional veterinary advice, diagnosis, or treatment. Always consult a qualified veterinarian before administering any medication to your pet. Find a vet near you →