Thyroid hormones

L-thyroxine is identical in structure and mode of action to the thyroxine (T4) secreted physiologically and present in mammals with a normally functioning thyroid gland. Thyroxine is metabolised mainly to tri-iodothyronine (T3). T4 and T3 have a large variety of biological effects throughout the body. They are essential for the regulation of basal metabolism, cardiac function and blood flow, lipid and carbohydrate metabolism. They are also essential for the normal growth and development of the neurological and skeletal systems.

There is considerable variation in the pharmacokinetics between individual dogs. After oral administration of the product to euthyroid, fasted dogs, tmax occurred at approximately 2.5 – 3 hours. The serum half-life of L-thyroxine was approximately 7 hours. Bioavailability was 22 %.

After repeated oral administration over 14 consecutive days at a dose rate of 40 μg/kg/day, there was no accumulation of L-thyroxine in serum. Concomitant administration of food with the product delays absorption and reduces the extent of absorption of L-thyroxine from the gastrointestinal tract by approximately 50 %. L-thyroxine is highly protein bound.

The major site of thyroxine (T4) metabolism is the liver. The main pathway for the metabolism of T4 is its conversion, by deiodination, to the active metabolite triiodothyronine (T3). Further deiodination of T4 and T3 leads to production of inactive compounds.

Excretion is mainly observed via biliary and, to a lesser extent, urinary routes.