Pharmacokinetics of total thyroxine after repeated oral administration of levothyroxine solution and its clinical efficacy in hypothyroid dogs.
BACKGROUND
Oral levothyroxine (l-T4 ) supplementation is commonly used to treat hypothyroid dogs.


OBJECTIVE
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.


METHODS
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
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.
Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands.
I C
IC
van Dijl
G
G
Le Traon
B D A M
BD
van de Meulengraaf
S
S
Burgaud
L J I
LJ
Horspool
H S
HS
Kooistra
D000284:Administration, Oral; D000818:Animals; D004283:Dog Diseases / Q000188:drug therapy*; D004285:Dogs; D005260:Female; D007037:Hypothyroidism / Q000188:drug therapy / Q000662:veterinary*; D008297:Male; D013974:Thyroxine / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics* / Q000627:therapeutic use; D016896:Treatment Outcome
D013974:Thyroxine
Canine; Hypothyroidism; Plasma profile; Thyroid hormone supplementation