Plasma adrenocorticotropin concentrations vary with assay used in horses with and without pituitary pars intermedia...

Plasma adrenocorticotropin (ACTH) concentration is measured to support a diagnosis of pituitary pars intermedia dysfunction (PPID). However, baseline ACTH and ACTH concentrations after IV administration of thyrotropin releasing hormone (TRH) vary with season and assay used. To compare two ACTH assays (Scantibodies immunoradiometric assay [IRMA] and Immulite 2000Xpi chemiluminescent immunometric assay [CIA]) for diagnosis of PPID, ACTH was measured before and 10 minutes after administration of TRH in three groups of horses: 1) four young horses (6 ± 3 years, no hypertrichosis and a pituitary gland [PG] histology score of 1-3/5); 2) eight aged horses (26 ± 6 years, no hypertrichosis and a PG histology score of 2-3/5); and 3) five PPID⁺ horses (27 ± 5 years, all with hypertrichosis and a PG histology score of 5/5). All horses underwent TRH stimulation in non-fall months with necropsy examination within 7 days. Baseline ACTH was elevated (>40 pg/mL) with IRMA in 0/4, 2/8, and 4/5 and with CIA in 0/4, 4/8, and 4/5 of young, aged, and PPID⁺ horses, respectively. After TRH administration, ACTH was elevated (>200 pg/mL) with IRMA in 0/4, 0/8, and 2/5 and with CIA in 1/4, 5/8, and 5/5 of young, aged, and PPID⁺ horses, respectively. Mean increases in ACTH after TRH administration were 13-, 18-, and 14-fold greater with CIA compared to IRMA in young, aged, and PPID⁺ horses, respectively. In conclusion, ACTH (or immunoreactive ACTH) concentration before and after TRH administration varies substantially with assay used and can affect PPID classification.