[Serum levels od D-thyroxine in euthyroidism and hypothyroidism (author's transl)]

Serum levels of D-thyroxine (D-T4) and L-thyroxine (L-T4) were determined by use of a newly developed method for the stereospecific determination of these isomers in 8 patients with diffuse non-toxic goiter and 2 hypothyroid patients following administration of increasing doses of D-thyroxine. During the 3-day observation period a significant decrease of serum-TSH takes place. D-T4 levels are highest 4 hours after administration of the tablets and approach basal values within 24 hours in euthyroid as well as hypothyroid patients. Serum half life of D-T4 was 5.7 hours on the average in the euthyroid group and was markedly prolonged in hypothyroidism. L-T4 levels show no significant changes. Since the D-T4 preparation used, contained only very little amounts of the L-isomer, it may be assumed that D-T4 exerts a direct suppressive effect on the pituitary TSH secretion, as evidenced by the observed significant decrease in basal and TRH-stimulated TSH serum levels.