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J Vet Intern Med. 2014 Jul-Aug;28(4):1229-34. doi: 10.1111/jvim.12363. Epub 2014 Apr 28.

Pharmacokinetics of total thyroxine after repeated oral administration of levothyroxine solution and its clinical efficacy in hypothyroid dogs.

Author information

1
Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.

Abstract

BACKGROUND:

Oral levothyroxine (l-T4 ) supplementation is commonly used to treat hypothyroid dogs.

OBJECTIVES:

Investigate the plasma profile and pharmacokinetics of total thyroxine (tT4 ) after PO administration of a l-T4 solution and its clinical efficacy in hypothyroid dogs.

ANIMALS:

Ten dogs with naturally occurring hypothyroidism.

METHODS:

After hypothyroidism diagnosis and supplementation with l-T4 solution PO q24h at 20 μg/kg BW for minimum 4 weeks, the plasma profile and pharmacokinetics of tT4 were determined over 34 hours and the clinical condition of the dogs was evaluated.

RESULTS:

Before dosing for pharmacokinetic evaluation, mean tT4 concentration was 23 ± 9 nmol/L. l-T4 was absorbed rapidly (tmax , 5 hours), reaching a mean maximal tT4 concentration of 56 ± 11 nmol/L. The apparent terminal half-life was 11.8 hours. Clinical signs of hypothyroidism improved or resolved in all dogs after 4 weeks of treatment. The dosage of 20 μg/kg PO q24h was judged appropriate in 5 dogs, and 4 dogs required slight increases (9-16%). Twice daily treatment, with a 30% increase in dosage, was necessary for 1 dog.

CONCLUSIONS AND CLINICAL IMPORTANCE:

The pharmacokinetics of l-T4 in hypothyroid dogs was similar to that reported in healthy euthyroid dogs. Clinical and hormonal responses to l-T4 solution were rapid in all dogs. The starting dosage of 20 μg/kg PO q24h was suitable for maintenance supplementation in 50% of the dogs, minor dosage modification was required in 4 other dogs, and treatment q12h was required in 1 dog.

KEYWORDS:

Canine; Hypothyroidism; Plasma profile; Thyroid hormone supplementation

PMID:
24773132
PMCID:
PMC4857956
DOI:
10.1111/jvim.12363
[Indexed for MEDLINE]
Free PMC Article

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