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

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Ziglioli V, Panciera D, Monroe E, et al.

 in Conference Proceedings. American College of Veterinary Internal Medicine 2016.

Many dogs are misdiagnosed with hypothyroidism because of the vague presenting signs and limitations of thyroid function tests, leading to inappropriate treatment with levothyroxine. During chronic supplementation, the hypothalamic-pituitary-thyroid axis (HPTA) is suppressed and may make it difficult to accurately determine thyroid function following withdrawal of levothyroxine. We sought to determine if the HPTA is suppressed following levothyroxine administration in euthyroid dogs and the time required for resolution of any suppression. We hypothesized that levothyroxine administration would suppress the HPTA in euthyroid dogs and that the HPTA would recover within 8 weeks in all dogs, regardless of the duration of treatment.

Twenty-eight healthy euthyroid dogs were administered levothyroxine (mean dose 0.024 mg/kg every 24 hours) for either 8 weeks (Group 1) or 16 weeks (Group 2). The dose of levothyroxine was adjusted weekly until a target total thyroxine (T4) serum concentration of 40–70 nmol/L was obtained 4–6 hours post pill. Serum concentrations of T4, free thyroxine (fT4) by equilibrium dialysis, thyrotropin (TSH), and 3,5,3′-triiodothyronine (T3) were measured every 4 weeks during supplementation and for 4 months after levothyroxine was discontinued. A mixed model ANOVA followed by Tukey’s procedure for multiple comparisons was used to compare thyroid function tests both within and between groups. The level of significance was set at < 0.05.

Mean serum concentrations of T4 and fT4 were significantly higher and TSH was significantly lower during levothyroxine administration compared to baseline in both groups. Mean serum concentrations of T4, fT4, and TSH

beginning 1 week after levothyroxine was discontinued were significantly different compared to values during levothyroxine administration but not compared to baseline values. When groups 1 and 2 were compared, there was no difference in mean T4, fT4, and T3 concentrations between groups during or after levothyroxine supplementation. Suppression of the HPTA occurred during levothyroxine supplementation, with mean serum T4, fT4, and TSH concentrations returning to the reference interval by 1 week after discontinuation in both groups. It appears that assessing thyroid function tests 1 week after long-term levothyroxine supplementation may reliably establish dogs as being euthyroid.