Effects of high doses of levothyroxine sodium on serum concentrations of triiodothyronine and thyroxine in horses.

The research explores how high doses of levothyroxine sodium (a thyroid hormone medication) given orally affects levels of thyroid hormones, triiodothyronine (T) and thyroxine (T), in the blood of healthy horses.

Methodology

• The study was conducted on 12 healthy adult horses.
• Initially, 10 horses were administered either water (as a control) or high doses of levothyroxine sodium – 240 mg (5 times the standard treatment) or 480 mg (10 times the treatment).
• Blood samples were taken at numerous intervals, ranging from half an hour to five days (120 hours) after the treatment, to measure the levels of triiodothyronine (T) and thyroxine (T) – the primary hormones produced by the thyroid gland. The changes in the concentration of these hormones over time and among the different drug dosages were then analyzed.
• Separately, three horses were given a daily dose of 480 mg of levothyroxine sodium for 14 days, and thyroxine concentration was measured on days 0, 14, 21, 28, and 35.

Findings

• The one-time administration of levothyroxine sodium resulted in variable yet significant increases in the concentration of both T and T hormones, up to 120 hours. However, except for a few cases, the hormone concentrations didn’t exceed the reference ranges for healthy horses.
• The 14-day prolonged administration of 480 mg levothyroxine sodium caused a 15-fold increase in thyroxine concentration on the final day of treatment. Nevertheless, this concentration decreased back to the initial day 0 values within 21 days after the hormone administration ceased.

Conclusion

• While high doses of levothyroxine sodium resulted in mild increases in thyroid hormone concentrations in healthy horses, the prolonged administration led to starkly increased hormone levels. However, these concentrations returned to baseline levels within 21 days of discontinuing the medication.
• This discovery suggests complex dynamics in how levothyroxine sodium is processed in horses, implying a possible saturation in thyroid hormone secretion in healthy horses.