Response of newborn foals with thyroid musculoskeletal disease to adrenocorticotrophic hormone (ACTH).

This research article investigates the adrenal gland functionality in newborn foals suffering from Thyroid Musculoskeletal Disease (TH-MSD), aiming to understand how thyroid function may influence the timing of horse birth. The study reveals that TH-MSD foals’ cortisol response to adrenocorticotropic hormone (ACTH) remains significant despite zero effect on adrenal cortical secretion, indicating thyroid function may contribute considerably to parturition timing.

Understanding the Role of Thyroid and Adrenal Glands in Equine Parturition

• While the exact mechanisms involved in horse birthing are not entirely understood, thyroid and adrenal glands are believed to be key regulators.
• Extended gestation periods observed in equine fetuses subjected to thyroidectomy suggest a correlation between thyroid gland function and parturition.
• Spontaneous diseases like TH-MSD and gestational fescue endophyte exposure also show association with delayed parturition, reinforcing the potential thyroid-parturition connection.

Characteristics of Thyroid Musculoskeletal Disease (TH-MSD)

• TH-MSD is characterized by overactive thyroid glands (histologically hyperplastic), dysfunctional development of cartilage and bone (chondro-osseous dysplasia and dysgenesis), physical deformities in limbs, underperforming thyroxine and triiodothyronine concentrations, and no response to thyroid-stimulating hormone.
• Foals born from mares that feed on endophyte-infected fescue grass show symptoms similar to those seen in TH-MSD foals, hinting at an underlying endocrine cause for the prolonged gestation periods.

Evaluating Adrenal Gland Function in TH-MSD Foals

• The objective of the research was to study the endocrine capability of the adrenal gland in TH-MSD foals, using ACTH response tests to compare adrenal function between healthy and diseased foals.
• The basal thyroxine concentrations differed significantly between healthy and TH-MSD foals, where TH-MSD foals showed lower thyroxine levels.
• Upon administering ACTH, a significant time effect was observed on cortisol concentrations, but no treatment effect was noted.
• Despite having a deficiency of thyroid hormones, ACTH administration did not significantly impact adrenal cortical secretion in TH-MSD foals.

Implications of the Research Findings

• The research findings suggest a primary role of thyroid function in managing the timing of equine parturition, either directly or indirectly through a different mechanism besides influencing adrenal response to ACTH.
• Understanding the endocrine interplay between the thyroid and adrenal glands in gestation management may pave the way for better therapeutic approaches for conditions like TH-MSD in the future.