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

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

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

Twenty-eight healthy euthyroid dogs.

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.

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 , 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).

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.