Role of L-thyroxine in nuclear thyroid hormone receptor occupancy and growth hormone production in cultured GC cells

The contribution of L-thyroxine (T₄) to nuclear thyroid receptor occupancy was studied in GC cells incubated with concentrations of 3,5,3′-triiodo-L-thyronine (T₃) and T₄ that resulted in free iodothyronine levels similar to those in serum of euthyroid rats. T₄ accounted for 5.4-10% of the occupied receptors: T₃ derived from T₄ [T₃(T₄)] and T₃ added to medium accounted for the remainder of receptor occupancy. Incubation with increasing medium free T₄ resulted in a progressive increase in the contribution of T₄ and T₃(T₄) to receptor occupancy. In incubations with 3.6-fold increased medium free T₄, T₄ accounted for 20.4%, and T₃(T₄) for 40.3% of receptor occupancy. These occupancy data and the experimentally determined K_a of thyroid receptor for T₃ and T₄ allowed calculation of nuclear free iodothyronine concentrations. Nuclear free T₃ was 3-6-fold greater than medium free T₃ and medium free T₄ was 12-19-fold greater than medium free T₄. When GC cells were incubated with decreased medium free T₃ and physiological medium free T₄, both nuclear receptor occupancy and growth hormone production decreased as well. However, a twofold increase in medium free T₄, in the presence of decreased medium free T₃, restored receptor occupancy and growth hormone production to or near control values. These findings establish a role for T₄ in addition to T₃(T₄) in nuclear receptor occupancy and biological activity in rat anterior pituitary tissue both in physiologic conditions and when medium free T₄ is raised. The findings may have relevance to the sick euthyroid thyroid syndrome in which free T₄ may be increased in some patients who have decreased serum free T₃.