Abstract
Summary
Data on the biological half-life (t 1/2) of L-thyroxine-I131 (L-T4-I131) in thyroidectomized, thyroidectomized thyroxine replaced, normal and methimazole treated female rats are presented. Rats thyroidectomized for 7 days had a significantly longer mean t1/2 of L-T4-I131 (P<.005) of 23.7 ± 0.7 hours than thyroidectomized rats which received a mean TSR level of replacement thyroxine of 1.0 μg L-T4/100 g bw and had a mean t1/2 of 17.8 ± 1.1 hours. These thyroxine replaced rats did not differ from intact controls which had a mean t 1/2 of 19.5 ± 2.2 hours. Hypothyroidism induced by subcutaneous injection of 400 μg methimazole/100 g bw/day for 30 days had no effect on the t1/2 of L-T4-I131 as shown by a mean t1/2 of 18.5 ± 2.1 hours. It was shown also that administration of the tracer dose of L-T4-I131 could be made by either subcutaneous or intracardial injection without effect on the t1/2 if proper intervals were maintained between time of injection and collection of the initial blood sample.
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