Levothyroxine Sodium
3,5,3',5'-tetra-iodo-L-thyronine sodium
Also known as: Soloxine · Synthroid · Levosyn · Thyro-Tabs · Thyrosyn · Leventa · Thyro-L · Levothroid · Levoxyl · Tirosint · T4 thyroxine sodium · levothyroxin natrium · levothyroxinum natricum · L-thyroxine sodium · tirossina · tiroxina sodica
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hypothyroidism | 20 micrograms/kg (0.02 mg/kg) body weight PO twice daily with a maximum dose of 0.8 mg twice daily | PO | q12h | — | 🌎 NA |
| Hypothyroidism | 10-22 micrograms/kg q12h or q24h | PO | q12h or q24h | — | 🌎 NA |
| Hypothyroidism | 0.02 mg/kg PO twice daily to start; (0.02-0.04 mg/kg PO once daily or, if necessary divided twice daily to maintain). Alternatively, give 0.5 mg/m2 | PO | q12h or q24h | — | 🌎 NA |
| Myxedema coma | 5 micrograms/kg IV q12h initially | IV | q12h | — | 🌎 NA |
- Hypothyroidism: Evaluate clinical response 4-8 weeks later and draw a T4 level 4-6 hours post dosing.
- Hypothyroidism: Maximum of 0.8 mg q12h. Reevaluate 1-2 months after initiating therapy.
- Hypothyroidism: 0.5 mg/m2 dosing may prevent hyperthyroid effects, particularly in large breed dogs.
- Myxedema coma: Oral administration may be poorly absorbed in this state.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hypothyroidism | 0.05-0.1 mg per cat PO once daily | PO | q24h | — | 🌎 NA |
| Post thyroidectomy | 0.1-0.2 mg (total dose) PO once daily beginning 24-48 hours post-op | PO | q24h | Several weeks or months | 🌎 NA |
- Hypothyroidism: Wait a minimum of 4-6 weeks to assess clinical response.
- Post thyroidectomy: Monitor T4 levels to determine when supplementation can be ceased.
Birds
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hypothyroidism | One 0.1 mg tablet in 30 mL-120 mL of water daily | PO | q24h | — | 🌎 NA |
- Hypothyroidism: Stir water and offer for 15 minutes and remove. Use high dose for budgerigars and low dose for water drinkers.
Reptiles
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hypothyroidism in tortoises | 0.02 mg/kg PO every other day | PO | q48h | — | 🌎 NA |
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Hypothyroidism | 10 mg in 70 mL of corn syrup once daily | PO | q24h | — | 🌎 NA |
| Adjunctive treatment of equine metabolic syndrome | 0.1 mg/kg PO once daily, initiate treatment at 48 mg/day... and then increase to 72 mg/day after 3 months if the horse remains obese | PO | q24h | 6 months | 🌎 NA |
- Hypothyroidism: Monitor T4 levels one week after initiation of therapy.
- Adjunctive treatment of equine metabolic syndrome: Taper off by administering 24 mg/day for 2 weeks and then 12 mg/day for 2 weeks.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Levothyroxine sodium is a synthetic, levo-isomer of thyroxine (T4), an endogenous hormone secreted by the thyroid gland. It is considered the gold standard for thyroid hormone replacement therapy in veterinary medicine, primarily used to treat hypothyroidism in dogs, and occasionally in other species.
Key Clinical Pearls:
- Prohormone Nature: T4 is largely a prohormone that is converted in peripheral tissues to the more biologically active T3.
- Bioavailability: Oral absorption can be highly variable among individual animals and can be affected by food. Fasting administration is often recommended for consistent absorption.
- Therapeutic Monitoring: Because of individual pharmacokinetic variability, therapeutic drug monitoring (TDM) of serum total T4 (TT4) is essential to tailor the dose to the individual patient.
Mechanism of action
Thyroid hormones regulate multiple metabolic processes, including fat, protein, and carbohydrate metabolism, and are critical for normal growth, development, and basal metabolic rate.
- Pathway: Exogenous levothyroxine (T4) is absorbed and distributed to peripheral tissues.
- Conversion: Inside cells, T4 is enzymatically deiodinated by 5'-deiodinase to triiodothyronine (T3), the primary active hormone.
- Receptor Binding: T3 enters the nucleus and binds to thyroid hormone receptors (TRs).
- Gene Transcription: The TR-T3 complex binds to thyroid response elements (TREs) on DNA → alters gene transcription → increases mRNA synthesis → increases protein synthesis, oxygen consumption, heart rate, and cellular metabolism.
Safety & warnings
Contraindications
- Acute myocardial infarction
- Thyrotoxicosis
- Untreated adrenal insufficiency
Adverse effects
- Tachycardia
- Polyphagia
- Polyuria/Polydipsia (PU/PD)
- Excitability
- Nervousness
- Excessive panting
- Apathy or listlessness (seen in some cats)
Precautions
Use with caution and at a lower initial dosage in patients with concurrent hypoadrenocorticism (treated), cardiac disease, diabetes, or in elderly patients. High fiber diets may reduce absorption. Hypothyroid dogs can have decreased GFR; restoration to a euthyroid state can increase GFR and reduce serum creatinine levels. Numerous drugs can affect thyroid function tests (T4, T3, TSH), so evaluate lab results accordingly.
Drug interactions
May decrease the metabolism of T4 to T3
May reduce levothyroxine absorption; separate doses by 4 hours
Increased risk for CNS stimulation and cardiac arrhythmias
Levothyroxine may increase requirements for insulin or oral agents
May reduce levothyroxine absorption; separate doses by 4 hours
Decreased conversion of T4 to T3
Potential for reduced digoxin levels
May reduce levothyroxine absorption; separate doses by 4 hours
May cause tachycardia and hypertension
Possible increased metabolism of thyroxine; dosage adjustments may be needed
Decreased conversion of T4 to T3
Possible increased metabolism of thyroxine; dosage adjustments may be needed
May increase levothyroxine requirements
May reduce levothyroxine absorption; separate doses by 4 hours
Levothyroxine can potentiate effects
Thyroid hormones increase the catabolism of vitamin K-dependent clotting factors that may increase the anticoagulation effects
Monitoring
- Clinical signs of resolution of hypothyroidism (e.g., improved energy, hair regrowth, weight loss)
- Signs of thyrotoxicosis/hyperthyroidism (tachycardia, panting, weight loss with polyphagia)
- Serum total T4 (TT4) levels (typically measured 4-6 hours post-pill for peak levels)
- Canine TSH (cTSH) if indicated
Pharmacokinetics
Half-life
Absorption
Wide variability from animal to animal.
Metabolism
Converted in peripheral tissues from T4 to T3.
Overdose
Chronic overdosage will produce signs of hyperthyroidism, including tachycardia, polyphagia, PU/PD, excitability, nervousness, and excessive panting. Some cats (approx. 10%) may exhibit 'apathetic' hyperthyroidism (listlessness, anorexia).
- Management of Chronic OD: Reduce dosage and/or temporarily withhold until signs subside.
- Acute Overdose: Less likely to cause severe thyrotoxicosis than chronic OD. Signs may include vomiting, diarrhea, hyperactivity to lethargy, hypertension, tachycardia, tachypnea, dyspnea, and abnormal pupillary light reflexes. In dogs, signs appear within 1-9 hours.
- Treatment of Acute OD: If ingestion occurred within 2 hours, use standard decontamination protocols (emetics, cathartics, activated charcoal). Provide supportive care (oxygen, artificial ventilation, beta-blockers like propranolol, fluids, dextrose, antipyretics).
Available products
Formulations
- Chewable tablets
- Oral solution
- Injection (lyophilized powder)
- Liquid-filled oral capsules
Veterinary
- Levothyroxine Sodium Tablets: 0.1 mg, 0.2 mg, 0.3 mg, 0.4 mg, 0.5 mg, 0.6 mg, 0.7 mg, 0.8 mg, 1 mg (Soloxine, Levosyn, Thyro-Tabs, Thyrosyn, Thyroxine-L, Thyrozine, Thyrokare)
- Levothyroxine Sodium Tablets Chewable: 0.1 mg to 0.8 mg (Canine Thyroid Chewable Tablets, Nutrived T-4, Heska Thyromed)
- Levothyroxine Oral Solution: 1 mg/mL (Leventa)
- Levothyroxine Sodium Powder: 0.22% (Equine Thyroid Supplement, Thyrozine Powder, Levoxine Powder, Thyro-L, Throxine-L Powder, Equi-Phar Thyrosyn Powder, Thyrokare Powder)
Human-labeled
- Levothyroxine Sodium Tablets: 0.025 mg to 0.3 mg (Synthroid, Levothroid, Levoxyl, Unithroid)
- Levothyroxine Sodium Liquid-filled Oral Capsules: 13 mcg to 150 mcg (Tirosint)
- Levothyroxine Powder for Injection lyophilized: 200 mcg & 500 mcg
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store at room temperature in tight, light-resistant containers. The injectable product should be reconstituted immediately before use; discard unused portions. Do not mix injection with other drugs or IV fluids. Aqueous solutions are reportedly unstable.
Client information
Important: Consistency is key! Give this medication at the same time every day, ideally on an empty stomach or consistently with the same type of food, as food can affect how much drug is absorbed.
- Lifelong Therapy: Hypothyroidism is typically a lifelong condition. Do not stop giving the medication without consulting your veterinarian.
- Monitoring: Your pet will need periodic blood tests to ensure the dose is exactly right. Too little won't fix the symptoms, and too much can cause side effects.
- Watch for Overdose Signs: Contact your veterinarian if your pet starts drinking or urinating excessively, panting heavily, acting nervous or hyperactive, or losing weight despite a ravenous appetite.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.