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

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 .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.