Thyroid-stimulating hormone: response test in healthy horses, and effect of phenylbutazone on equine thyroid hormones.

This research investigates the variations in thyroid hormone levels in adult horses, and the effect of phenylbutazone on these hormones. It was found that certain hormones show diurnal fluctuations, with an increase in TSH leading to spikes in T3 and T4 levels. Administration of phenylbutazone resulted in an overall decrease in base level T4 and T3, but an increased response to TSH injections.

Natural Fluctuation of Thyroid Hormones

• The study observed a mild diurnal variation in the levels of thyroxine (T4), a thyroid hormone, in equine plasma. This means these hormone concentrations changed over the course of the day, tending to be higher from 5 PM to 8 PM instead of at 8 AM. However, such variations didn’t appear in the levels of another thyroid hormone triiodothyronine (T3).

Response to Thyroid-Stimulating Hormone

• When adult horses were given varying units of thyroid-stimulating hormone (TSH), their plasma T4 and T3 levels increased similarly across dosage levels. The most notable change was seen around 6 to 12 hours after TSH administration, where T4 levels typically reached approximately twice the baseline value.
• Meanwhile, T3 showed the greatest deviation from the baseline at around 1 to 3 hours after TSH administration. The degree of this increase, however, varied substantially among the subjects.

Suggested Testing Method and Phenylbutazone Treatment Impact

• The researchers suggested a method for conducting the equine TSH-response test. It involves the preparation and storage of plasma or serum samples at specific times after TSH injection, and subsequent examination using radioimmunoassay.
• Treatment of the horses with phenylbutazone for 5 days resulted in a significant decrease in baseline T4 and T3 levels. Interestingly, horses administered with phenylbutazone had a sharper increase in T4 levels than control horses 6 hours after TSH injection.