Thyroxine (T4) metabolism in an athyreotic patient who had taken a large amount of T4 at one time

As we had an opportunity to take blood samples from a totally thyroidectomized patient who had attempted suicide by taking 2,000 mu g of Levothyroxine (L-T-4), the serum levels of thyroid hormones were sequentially measured to investigate the metabolism of circulating thyroid hormones in an athyreotic human. The serum concentrations of most thyroid hormones reached a peak on the second day, but the serum T-3 level showed a peak one day later. The maximum concentrations of T-4 (315 mu g/l), FT4 (48.8 ng/l) and rT(3) (0.80 mu g/l) were very high, while the peak T-3 level (1.92 mu g/l) did not exceed the upper limit of the normal range. The serum T-4 and rT(3) levels returned to their normal range 13-17 days after the suicide attempt. The TSH level was suppressed rapidly and reached its nadir (0.044 mU/l) on the 6th day. During this period, the T-1/2 and MCR of serum T-4 were 10.4 days and 0.64 l/day, respectively, which values were almost equivalent to those observed during 15 days after discontinuation of the maintenance L-T-4 therapy. In summary, the oral intake of a large amount of L-T-4 at one time does not induce a proportional increase in the T-3 level in an athyreotic person. The MCR of serum T-4 is decreased and the T-1/2 Of serum T-4 is prolonged, probably due to the lack of intrathyroidal deiodination. These findings support the conclusion that the D1 activity in the thyroid is one of the major determinants in the metabolic clearance of serum T-4.