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.
Copyright © 2017 The Authors. Journal of Veterinary Internal Medicine published by Wiley Periodicals, Inc. on behalf of the American College of Veterinary Internal Medicine.