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J Vet Intern Med. 2017 May;31(3):705-710. doi: 10.1111/jvim.14711. Epub 2017 Apr 22.

Effects of Levothyroxine Administration and Withdrawal on the Hypothalamic-Pituitary-Thyroid Axis in Euthyroid Dogs.

Author information

1
Department of Small Animal Clinical Sciences, Virginia-Maryland College of Veterinary Medicine, Blacksburg, VA.
2
Department of Biomedical Sciences and Pathobiology, Virginia-Maryland College of Veterinary Medicine, Blacksburg, VA.
3
Diagnostic Center for Population and Animal Health Michigan State University, Lansing, MI.

Abstract

BACKGROUND:

Chronic supplementation can suppress the hypothalamic-pituitary-thyroid axis (HPTA) and make it difficult to assess thyroid function after withdrawal of levothyroxine.

OBJECTIVES:

To determine whether the HPTA is suppressed after levothyroxine administration in euthyroid dogs and the time required for resolution of any suppression.

ANIMALS:

Twenty-eight healthy euthyroid dogs.

METHODS:

A prospective, randomized study administering levothyroxine to euthyroid dogs for 8 weeks (group 1) or 16 weeks (group 2). Serum concentrations of total thyroxine (T4 ), free thyroxine (fT4 ) by equilibrium dialysis, thyroid stimulating hormone; thyrotropin (TSH), and 3,5,3'-triiodothyronine (T3 ) were measured every 4 weeks during supplementation and for 16 weeks after levothyroxine was discontinued.

RESULTS:

Mean serum concentrations of T4 and fT4 were significantly higher (P < .0001) and TSH was lower (P < .0001) in all dogs during levothyroxine administration compared to baseline. Mean serum concentrations of T4 , fT4, and TSH in both groups, beginning 1 week after levothyroxine was discontinued, were significantly different (P < .01) compared to values during levothyroxine administration but not compared to baseline values (P > .3).

CONCLUSIONS AND CLINICAL IMPORTANCE:

Assessing thyroid function tests 1 week after cessation of levothyroxine at 26 μg/kg once a day for up to 16 weeks will provide an accurate assessment of thyroid function in healthy euthyroid dogs.

KEYWORDS:

Canine; Endocrinology; Hypothyroidism; Thyroid function tests

PMID:
28432797
PMCID:
PMC5435074
DOI:
10.1111/jvim.14711
[Indexed for MEDLINE]
Free PMC Article

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