Calculate the correct Levothyroxine (T4) dose for your hypothyroid dog.
Dosage Calculator
Enter your dog's weight to calculate the recommended dose.
Calculated Dose
Per dose ()
Per dose in mcg
Nearest tablet size
Check T4 levels 4–6 hrs post-pill at 4–8 weeks.
Enter your dog's weight above
⚕️ Levothyroxine dosing must be individualised and monitored with regular T4 blood testing. This calculator provides a starting reference only. All thyroid treatment must be overseen by a veterinarian.
Levothyroxine sodium (L-T4) is the standard treatment for canine hypothyroidism, a condition where the thyroid gland produces insufficient thyroid hormone. It is sold under brand names including Thyforon, Soloxine, and Forthyron (canine-specific formulations are preferred over human products).
Starting Dose
The recommended starting dose is 0.02 mg/kg (20 mcg/kg) given twice daily, or 0.04 mg/kg once daily. Twice-daily dosing generally produces more consistent thyroid levels and is preferred. Maximum starting dose is typically 0.8 mg/day regardless of body weight.
Monitoring & Adjustment
Serum total T4 levels should be measured 4–6 hours post-pill at 4–8 weeks after starting or changing dose. Target post-pill T4 is in the upper half of the reference range. Dose adjustments are made in 10–25% increments based on results.
Available Tablet Strengths
Canine levothyroxine tablets are available in 0.1 mg, 0.2 mg, 0.3 mg, 0.4 mg, 0.5 mg, 0.6 mg, 0.7 mg, and 0.8 mg strengths. Use veterinary formulations where possible as bioavailability can differ from human products.
🦋 Thyroid Hormone Replacement
Levothyroxine Dog Dosage Calculator
Precise levothyroxine (Soloxine, Thyro-Tabs, Thyroxine-L) dosing for dogs with hypothyroidism — covering starting dose, twice-daily vs once-daily protocols, tablet selection across all strengths, and TT4 monitoring targets with dose adjustment guidance.
ℹ️
Lifelong daily therapy — consistent timing is critical. Give levothyroxine at the same time each day, 30–60 min before the first meal for best absorption. TT4 monitoring should be performed 4–6 hours post-pill (peak) at 4–8 weeks after starting or any dose change. Bioavailability varies significantly between branded and generic formulations — do not switch brands without rechecking TT4.
💊
About Levothyroxine Sodium (Soloxine / Thyro-Tabs / Forthyron)
Levothyroxine (T4) is the synthetic form of the primary thyroid hormone thyroxine. It is converted peripherally to the more biologically active triiodothyronine (T3) by deiodinase enzymes. Thyroid hormones regulate metabolism, thermogenesis, protein synthesis, cardiovascular function, and neurological development.
Primary Uses in Dogs
Hypothyroidism (primary — most common endocrine disorder in dogs after diabetes mellitus). Signs include weight gain, lethargy, cold intolerance, poor coat quality, hyperlipidaemia, and bradycardia. Lifetime therapy required in virtually all cases.
Dosing Quick Reference
Indication
Dose
Frequency
Standard starting dose
0.02 mg/kg (20 mcg/kg)
q12h PO
Once-daily option (some dogs)
0.02 mg/kg
q24h PO — less consistent
Giant breeds (dose cap)
Max 0.8 mg/dose
q12h
Common Side Effects
Signs of overdose: panting, excessive thirst/urination, weight loss, tachycardia, hypertension
GI upset at high doses
Behavioural changes (hyperactivity, anxiety) with excessive dosing
Cardiac arrhythmias if overdosed in dogs with underlying heart disease
Monitoring
Post-pill T4 at 4–6h post-dose at weeks 4–8, then every 6 months once stable. Fasting lipid panel (cholesterol, triglycerides) as part of monitoring — hyperlipidaemia should normalise with adequate supplementation. ECG if cardiac signs develop.
⚠️ Monitor post-pill T4 (TT4) 4–6 hours after the morning dose at the 4–8 week recheck — target 25–45 nmol/L (2–3.5 mcg/dL). Generic preparations may not be bioequivalent to branded products — stick with one formulation once stabilised. Calcium, aluminium, and iron supplements reduce absorption — administer levothyroxine separately from these.
What Is Levothyroxine for Dogs?
Hypothyroidism is the most common endocrine disorder in dogs, resulting from immune-mediated destruction (lymphocytic thyroiditis) or idiopathic follicular atrophy of the thyroid gland in ~95% of cases. Treatment involves lifelong supplementation with synthetic levothyroxine (L-thyroxine, T4), which is converted peripherally to the biologically active T3 (triiodothyronine) by tissue deiodinases throughout the body.
Canine levothyroxine products include Soloxine, Thyro-Tabs (Vet), Thyroxine-L, and human generics. Dogs require significantly higher doses per kg than humans due to faster metabolism and greater GI absorption variability. Capsules/tablets are available from 0.1 mg (100 mcg) to 0.8 mg (800 mcg).
Key Pharmacokinetics in Dogs
Oral bioavailability: 10–50% (highly variable; food reduces absorption by ~20–40%)
Reduce dose 20–30%; assess for hyperthyroid signs; recheck 4 weeks
Frequently Asked Questions
Dogs metabolise levothyroxine much faster than humans. The elimination half-life in dogs is only 10–16 hours (versus ~7 days in humans), meaning dogs clear the drug much more rapidly and require higher doses per kg given twice daily. Additionally, GI absorption of levothyroxine in dogs is more variable (10–50%) and lower than in humans (~80%). The combination of faster elimination and lower/more variable absorption means dogs require 20–44 mcg/kg/day versus the human equivalent of approximately 1.6 mcg/kg/day.
For the most useful TT4 monitoring result, blood should be collected 4–6 hours after the morning levothyroxine dose. This coincides with peak plasma concentrations. Your dog should receive its normal morning dose with a small amount of food (or fasted if that’s the normal routine), then come to the clinic 4–6 hours later. A peak TT4 of 2.5–4.5 mcg/dL at this time indicates appropriate replacement. Pre-pill TT4 (trough) can also be measured — target >1.0 mcg/dL — but peak monitoring is preferred for initial dose titration.
Yes — human levothyroxine (Synthroid, Levoxyl, Euthyrox, generic) is the same active ingredient and can be used in dogs. However, bioavailability can differ between brands and between human and veterinary formulations. If you switch from Soloxine/Thyro-Tabs to a human generic or vice versa, always recheck TT4 levels 4–8 weeks after switching as your dog may need a dose adjustment. Human tablets often come in a wider range of strengths (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 mcg) which can be useful for fine-tuning doses in small dogs.
Clinical improvement typically follows a predictable timeline: increased energy and activity levels are often seen within 1–2 weeks; weight loss and improved coat quality takes 4–8 weeks; neurological signs (facial nerve paralysis, ataxia) may take months to partially or fully resolve. Skin changes (hyperpigmentation, scaling, hair regrowth) show improvement over 2–3 months. If the dog is being over-treated, signs of hyperthyroidism may appear: increased thirst/urination, weight loss despite good appetite, panting, excitability, and tachycardia.
There is some evidence and clinical reports of a link between hypothyroidism and aggression in dogs — a syndrome sometimes termed “hypothyroid aggression.” Proposed mechanisms include altered serotonin metabolism, cognitive dullness, and increased irritability. However, the relationship is debated in the veterinary literature and confirmed thyroid-related aggression is rare. Hypothyroidism should always be on the differential list for new-onset behaviour changes in middle-aged dogs, but levothyroxine should not be started empirically without confirmed diagnosis via TT4 and TSH testing — treating euthyroid dogs with levothyroxine can cause iatrogenic hyperthyroidism.
References
Plumb DC. Plumb’s Veterinary Drug Handbook, 9th ed. Wiley-Blackwell; 2018.
Dixon RM, et al. Epidemiological, clinical, haematological and biochemical characteristics of canine hypothyroidism. Vet Rec. 1999;145(17):481-487.
Panciera DL. Hypothyroidism in dogs: 66 cases. J Am Vet Med Assoc. 1994;204(5):761-767.