Technical Manager Boehringer Ingelheim Animal Health USA Inc. Stewartsville, Missouri, United States
Abstract: Hypothesis: This study evaluated a hypothesis that the thyrotropin releasing hormone (TRH) stimulation procedure repeated at weekly intervals would yield reproducible diagnosis of Pituitary Pars Intermedia Dysfunction (PPID). Understanding TRH-stimulation repeatability is useful if TRH-stimulation testing is initiated but unable to be completed or when PPID research protocols require multiple days of testing. The protocol was approved by the Institutional Animal Care and Use Committee. Animals: Five PPID-positive and 5 PPID-negative horses were used.
Methods: Basal blood samples were collected, then 1 mg TRH given i.v., and blood collected 10 and 30 min after TRH-stimulation, then repeated on days 7, 14, and 21. Immulite assay analyses of T0-ACTH, T10-ACTH, T30-ACTH on Day 0 vs subsequent days were compared using a mixed model with repeated measures, Pearson’s correlation coefficients, and Bland-Altman plots.
Results: There was no Day effect on T0-ACTH or T30-ACTH in PPID-Negative or PPID-Positive horses, or T10-ACTH in PPID-Negative horses. In PPID-Positive horses, T10-ACTH was lower on Day 14 (P < 0.036) than Days 7 and 21. While statistically significant, these differences were not clinically relevant, as ACTH remained above the PPID-Positive diagnostic cutoff. Bland-Altman plots indicated a Day bias in PPID-Negative horses of -1.97, -0.2, and 2.5 pg/mL at T0-ACTH, T10-ACTH, and T30-ACTH respectively; Day bias in PPID-Positive horses was 44.3, 8.7, and 62 pg/mL at T0-ACTH, T10-ACTH, and T30-ACTH respectively. The Immulite inter-assay CV was 9.3%, accounting for observed Day biases. Conclusions and clinical importance: TRH-stimulation test repeated at weekly intervals for 21 days provided consistent results.
