A comparison of changes in plasma thyrotropin beta- and alpha-subunits, and mouse thyrotropic tumor thyrotropin beta- and alpha-subunit mRNA concentrations after in vivo dexamethasone or T3 administration.

Dexamethasone, like T3, inhibits the production of TSH. T3 inhibits TSH synthesis by reducing transcription of the genes encoding TSH-beta and alpha-subunits. Little information is available concerning the effects of dexamethasone on the individual subunits, or the combined effects of dexamethasone with T3. In a preliminary study, hypothyroid mice bearing the thyrotropic tumor TtT 97 were treated with 25, 250, or 500 micrograms dexamethasone ip daily for ten days. Plasma levels of TSH and its subunits were unchanged after 25 micrograms of dexamethasone and maximally suppressed after 250 micrograms. Plasma TSH was reduced to 51% (P less than .02), free TSH-beta to 54% (P less than .01), and alpha-subunit to 62% (P less than .001) of control values. In two similar experiments hypothyroid mice bearing TtT 97 were treated with dexamethasone (250 micrograms), T3 (0.5 or 1 microgram), or both dexamethasone with T3 for 10 days. Total alpha-subunit and TSH-beta were calculated by adding 1/2 TSH + free subunit concentrations. In the experiment using 1 microgram of T3, total plasma alpha-subunit was reduced by dexamethasone to 72%, by T3 to 45% (P less than .02), and by combined treatment to 23% (P less than .01) of control values. In the experiment using 0.5 microgram of T3, total plasma alpha-subunit was reduced by dexamethasone to 66% (P less than .05), by T3 to 67% (P less than .05), and by combined treatment to 46% (P less than .02) of control values.(ABSTRACT TRUNCATED AT 250 WORDS)