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Introduction PPID affects 20% of horses >15 years, with development of hypertrichosis, muscle wasting, polyuria/polydipsia and immune dysfunction. Elevated basal ACTH concentration is commonly used for diagnosis, but thyrotropin releasing hormone (TRH)-stimulated ACTH concentration has greater sensitivity, especially in detecting early disease. During the transition to obvious clinical disease, ACTH concentrations may be equivocal. Materials and methods Basal and TRH-stimulated (TRH-stim) ACTH concentrations were measured, and clinical signs recorded throughout a 3.5-year period on 7 horses identified as equivocal for PPID based on locally derived seasonally adjusted diagnostic-cut off values. Ethics approval: SVS/562/18. Results No horses demonstrated clinical signs of PPID at the beginning of the study. Hypertrichosis developed and basal ACTH increased in 2 horses, with 1/11 negative basal ACTH results in 2018 and subsequent positive results prior to normalisation with pergolide treatment. A gelding with 3/12 negative basal ACTH results in 2018, and variable TRH-stim results developed mild hypertrichosis. A mare with no clinical signs had elevated basal ACTH in winter 2019 and elevated TRH-stim ACTH in autumn of 2018, 2020 and 2021. A mare with no clinical signs had consistently elevated TRH-stim ACTH. A mare with no clinical signs had intermittently elevated basal ACTH but consistently elevated TRH-stim ACTH in February/March. A gelding with no clinical signs had intermittent elevated basal and TRH-stim ACTH and grade 2 pituitary hyperplasia at necropsy. Relevance to clinical practice Horses with subclinical PPID may have equivocal or intermittently negative results, but TRH stimulation testing in February or March identifies most transitional cases. [ABSTRACT FROM AUTHOR]