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 (T), free thyroxine (fT) by equilibrium dialysis, thyroid stimulating hormone; thyrotropin (TSH), and 3,5,3'-triiodothyronine (T) were measured every 4 weeks during supplementation and for 16 weeks after levothyroxine was discontinued. RESULTS: Mean serum concentrations of Tand fTwere significantly higher (P < .0001) and TSH was lower (P < .0001) in all dogs during levothyroxine administration compared to baseline. Mean serum concentrations of T, fTand 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 &#x3bc;g/kg once a day for up to 16 weeks will provide an accurate assessment of thyroid function in healthy euthyroid dogs.