Enhancement of somatomedin titers of normal and hypopituitary sera by addition of L-triiodothyronone in vitro at physiological concentrations.

Somatomedin potencies of sera were assayed by following sulfation of mucopolysaccharides in chick embryo sterna in vitro. Apparent potencies of sera from hypophysectomized rats, maintained on a low-iodine diet, were restored to levels above normal by addition to the incubation medium of L-triiodothyronine at 10-7 mol/liter of serum. The potencies of normal rat, human, and fetal calf sera were raised 1.3- to 3-fold by addition of triiodothyronine at 10-9-10-7 mol/liter of serum. L-Thyroxine was about 10 times less effective than triiodothyronine. Triiodothyronine alone did not stimulate sulfation to nearly the same extent as triiodothyronine plus serum, even at higher concentrations. Serum could not be substituted in this system by any of six purified hormones, nor by trace metals or amino acids not included in the incubation mixture. It is concluded that triiodothyronine, in combination with a factor in serum, causes a rapid stimulation of sulfation in chick embryo cartilage, and that thyroid hormones may be involved in the action of normal serum on this tissue.