An in vitro test system for thyroid hormone action.

An in vitro bioassay for the biological action of thyroid hormones has been developed. The test is based on a dose-dependent increase in growth rate and cell activation of the rat pituitary tumor cell line GH3. This cell line is dependent on thyroid hormones for cell division when plated at low density in serum-free medium. The measurement of the cell-stimulating effect of L-triodothyronine (T3) is based on the capacity of mitochondrial enzymes of viable cells to transform the MTT tetrazolium salt into MTT formazan. A dose-dependent response of the MTT signal can be observed within a range from 0.075 to 1 nM T3 in the culture medium. The test is performed in 96-well microplates and allows screening of compounds which may interfere with the action of thyroid hormones.