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

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(4)), free thyroxine (fT (4)) by equilibrium dialysis, thyroid stimulating hormone; thyrotropin (TSH), and 3,5,3′‐triiodothyronine (T(3)) were measured every 4 weeks during supplementation and for 16 weeks after levothyroxine was discontinued. RESULTS: Mean serum concentrations of T(4) and fT (4) were significantly higher (P < .0001) and TSH was lower (P < .0001) in all dogs during levothyroxine administration compared to baseline. Mean serum concentrations of T(4), fT (4,) 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.