The effect of trilostane treatment on circulating thyroid hormone concentrations in dogs with pituitary-dependent hyperadrenocorticism

OBJECTIVE: To assess thyroid hormone levels in hyperadrenocorticoid dogs before and after therapy with trilostane, a reversible inhibitor of steroidogenesis. METHODS: Serum total thyroxine, free thyroxine and endogenous canine thyroid-stimulating hormone concentrations were measured in 20 dogs with spontaneously occurring hyperadrenocorticism before and six months after successful treatment with trilostane. RESULTS: Fourteen dogs demonstrated an increase in thyroxine following trilostane treatment; however, this was not significant (P=0.108). Fourteen dogs demonstrated an increase in canine thyroid-stimulating hormone concentrations with trilostane therapy (P=0.006). Of the 14 dogs that demonstrated an increase in thyroxine concentrations following therapy, 10 also showed an increase in canine thyroid-stimulating hormone concentrations. Before treatment, free thyroxine values were within, above and below the reference range in 10, six and two dogs, respectively. Sixteen of 18 dogs had free thyroxine values within the reference range after treatment, with 11 dogs showing a decrease in free thyroxine levels following therapy (P=0.029). CLINICAL SIGNIFICANCE: While treatment with trilostane did not induce a significant change of thyroxine concentrations, there was a significant increase in canine thyroid-stimulating hormone concentrations following treatment, a finding that supports thyroid-stimulating hormone suppression as one of the factors that contributes to the effects of glucocorticoids on the hypothalamic-pituitary-thyroid axis. The significant elevation in free thyroxine values following treatment with trilostane was unexpected and did not support the findings of previous studies in this area.