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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

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

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

Frequency

SID to BID

Max dose

0.2 mg/dose; 0.4 mg/day

Duration: Life-long if persistent

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 →

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

Educational reference only

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.

Educational reference only

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.

Educational reference only

References

The PetCare.AI drug reference is built from 13 authoritative sources cited across 580 drug monographs.

Textbooks & handbooks — 5

Plumb's Veterinary Drug Handbook
Withrow & MacEwen's Small Animal Clinical Oncology
Merck Veterinary Manual
NRC Nutrient Requirements of Dogs and Cats
Veterinary Pharmacology and Therapeutics (Riviere & Papich)

Clinical guidelines & consensus — 4

ASPCA Animal Poison Control Center Guidelines
AAHA Diabetes Management Guidelines
ASPCA Poison Control Guidelines
RECOVER CPR Guidelines

Journals & peer-reviewed studies — 2

EPIC Study (J Vet Intern Med 2016)
JVIM FAT CAT Study

Regulatory & approvals — 1

CDSCO Veterinary Drug Approval Registry (1969–2026)

Databases — 1

Washington State University VCPL MDR1 Database