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Levothyroxine dose for dogs and cats, checked by post-pill T4
Levothyroxine for a hypothyroid dog is 0.02 mg/kg (20 µg/kg) by mouth once daily or 0.01 mg/kg twice daily, and the Vetinuity levothyroxine dose table below sets the next dose from a total T4 drawn 4 to 6 hours after the pill once 4 weeks have passed.
Levothyroxine doses at a glance
| Species | Use | Dose | Route and interval |
|---|---|---|---|
| Dog | Hypothyroidism, tablets | 0.02 mg/kg once daily, or 0.01 mg/kg twice daily | PO, maximum about 0.8 mg per dose |
| Dog | Hypothyroidism, oral solution | 0.02 mg/kg | PO once daily |
| Dog | Heart disease or older dog | Start at a quarter to half the dose | PO, increase over 2 to 4 weeks |
| Cat | Iatrogenic hypothyroidism after treatment | 0.05 to 0.1 mg per cat | PO once daily |
Vetinuity levothyroxine dose calculator
Once or twice daily
Dogs were treated twice daily for years, but a liquid levothyroxine given once daily at 20 µg/kg brought clinical signs and T4 under control in a multicenter trial1. Tablets are often started twice daily and moved to once daily once the dog is stable, provided the post-pill T4 stays in range2. Large dogs reach the 0.8 mg per dose ceiling well before their weight-based number, and are dosed to that ceiling and their T4.
Timing the post-pill T4
The monitoring sample is a total T4 drawn 4 to 6 hours after the dose, at least 4 weeks after starting or after a change, when the level has settled2. The target is the upper half of the reference interval or slightly above it. A peak T4 well above the interval with signs of excess, such as panting, weight loss or restlessness, calls for a dose reduction; a peak inside the lower half with persistent signs calls for an increase of about 25 percent3.
Drugs and illness that move T4
Glucocorticoids, phenobarbital and nonthyroidal illness all lower total T4 without true thyroid failure, which can mislead both the diagnosis and the monitoring. An anti-inflammatory course of prednisone measurably lowered T4 in treated hypothyroid dogs, so a low post-pill value during steroid therapy is interpreted with care4.
Cats after treatment for hyperthyroidism
Cats rarely develop spontaneous hypothyroidism, but radioiodine or an overdose of methimazole can cause it. In an azotemic cat, a low T4 with a raised TSH warrants levothyroxine, since restoring thyroid function supports kidney function5.
Related values on this site: Creatinine, ALP.
Frequently asked questions
When is the post-pill T4 drawn?
Four to six hours after the dose, at least 4 weeks after starting or changing the dose2.
Can levothyroxine be given once a day?
Yes. Once-daily dosing at 20 µg/kg controlled hypothyroidism in a multicenter trial of a liquid formulation1.
Why is a dog's T4 low on levothyroxine while it is taking prednisone?
Glucocorticoids lower total T4 on their own, so the value can understate thyroid status during a steroid course4.
Sources
- Le Traon G, Brennan SF, Burgaud S, et al. Clinical evaluation of a novel liquid formulation of L-thyroxine for once daily treatment of dogs with hypothyroidism. J Vet Intern Med. 2009; 23(1):43-9. PMID: 19175719. DOI: 10.1111/j.1939-1676.2008.0236.x.
- Dixon RM, Reid SW, Mooney CT. Treatment and therapeutic monitoring of canine hypothyroidism. J Small Anim Pract. 2002; 43(8):334-40. PMID: 12201441. DOI: 10.1111/j.1748-5827.2002.tb00082.x.
- Bugbee A, Rucinsky R, Cazabon S, et al. 2023 AAHA Selected Endocrinopathies of Dogs and Cats Guidelines. J Am Anim Hosp Assoc. 2023; 59(3):113-135. PMID: 37167252. DOI: 10.5326/JAAHA-MS-7368.
- O'Neill SH, Frank LA, Reynolds LM. Effect of an anti-inflammatory dose of prednisone on thyroid hormone monitoring in hypothyroid dogs. Vet Dermatol. 2011; 22(2):202-5. PMID: 21070400. DOI: 10.1111/j.1365-3164.2010.00932.x.
- Carney HC, Ward CR, Bailey SJ, et al. 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. J Feline Med Surg. 2016; 18(5):400-16. PMID: 27143042. DOI: 10.1177/1098612X16643252.