Studies were conducted to determine if brief exposure, in utero, to high levels of T 4 or to the synthetic thyromimetic agent 3, 5-dimethyl-3′-isopropyl-L-thyronine (DIMIT) can produce permanent disruption of the thyroid control system in a manner analogous to the changes in the “set point” reported to occur due to neonatal T 4 exposure in the “neo-T 4 syndrome”. If such a change were to occur, it could explain the persistent thyroid disturbances seen in the progeny of hypothyroid mother rats. These latter progeny are exposed in utero to both low and high serum T 4 levels.