Severe Thyrotoxicosis Does Not Accelerate 1a-Hydroxylation of 25-Hydroxyvitamin D3 in Dogs. Experimental Study

Two healthy dogs weighing 18 kg and 13 kg each received an intravenous injection of 7 μg/kg 25-hydroxyvitamin D3 (25OHD3). Subsequently, they were blood-sampled in order to determine the plasma levels of 25(OH)D3 over 4-hourly time intervals and for a time period of 24 hours. After a period of 18 days since the last blood sampling, the animals were brought to a hyperthyroid state and the intravenous injection of 7 μg/kg 25OHD3 was repeated. Blood sampling was performed every 4 hours and over a time period of 24 hours in order to determine the levels of 25OHD3. The graphic plotting of plasma levels of 25OHD3 in the euthyroid state did not differ from that in the hyperthyroid state. This finding in dog animal experimentation is indicative that the increased levels of thyroid hormones did not affect the activity of CYP27B1 and CYP24A1 enzymes that are related to the catabolism of 25OHD3 over a minimum of 24 hours period.