Evaluation of seasonal influences on adrenocorticotropic hormone response to the thyrotropin-releasing hormone stimulation test and its accuracy for diagnosis of pituitary pars intermedia dysfunction.

The research investigated the seasonal effects on how adrenocorticotropic hormone (ACTH) responds to thyrotropin-releasing hormone (TRH) stimulation test, and how this affects diagnosis of Pituitary pars intermedia dysfunction (PPID), a neurodegenerative disorder in horses, using the TRH test.

Background of the study

• More than 20% of older horses are affected by an age-related neurodegenerative disorder called Pituitary pars intermedia dysfunction (PPID).
• Currently, there is a need for improved endocrine tests to detect this disorder early on, with the Thyrotropin-Releasing Hormone (TRH) stimulation test recommended for diagnosing mild or early cases.
• The TRH stimulation test however is not recommended for year-round use because of its significant seasonal variability.

Purpose of the study

• The study sought to investigate the effects of month and season on the response of Adrenocorticotropic Hormone (ACTH) to TRH stimulation.
• Another objective was to define monthly cut-offs for diagnosing PPID.
• For this, the researchers created three groups of horses: control, subclinical PPID, and clinical PPID, which they arrived at by considering clinical history, examination findings, and endocrine test results.

Procedure and Findings

• A TRH stimulation test was administered to these horses monthly over a year’s period.
• The research found that ACTH concentrations triggered by TRH stimulation were lowest from February to May and highest from August to October.
• Generally, both basal and 30 min post-TRH ACTH had higher specificity than sensitivity, meaning it’s more effective at correctly identifying horses without PPID than identifying those with it.
• However, the TRH stimulation had improved diagnostic accuracy compared to basal ACTH, though its sensitivity was not significantly greater throughout the year.

Implications of the study

• The study suggests that TRH stimulation tests yielded more positive results than basal ACTH in the subclinical group but not much in the clinical PPID group.
• The large differences between the cut-offs that maximised sensitivity or specificity for TRH-stimulated ACTH underlined the importance of considering clinical presentation alongside test results in diagnostic decision-making.