Nuclear thyroxine receptors and cellular metabolism of thyroxine in obese subjects before and after fasting.

Nuclear binding of thyroxine (T4), cellular deiodination of T4 and nuclear accumulation of endogenous triiodothyronine (T3) were investigated in mononuclear blood cells in 5 obese persons before and after 7 days of total fast. Serum-free T4, serum-free T3 and serum thyrotropin (TSH) did not differ between lean persons and obese fed subjects, but fasting induced a significant decrease of serum-free T4 and serum-free T3 (T4 from 16.6 to 14.4 pmol/l, T3 from 5.7 to 3.2 pmol/l). The maximal specific binding capacity (MBC) for T4 was decreased in fed obese subjects (MBC = 0.6 fmol T4/100 micrograms DNA, p less than 0.05) compared to normal weight persons (MBC = 1.3 fmol T4/100 micrograms DNA), whereas the equilibrium association constant (ka) did not differ. During complete fast, the MBC increased (MBC = 1.5 fmol T4/100 micrograms DNA, p less than 0.05). Neither total deiodination of T4 nor endogenous nuclear T3 accumulation differed between the groups suggesting that cellular T4 to T3 conversion is independent of the nutritional state. We suggest that the observed increase in nuclear T4 receptor capacity may compensate the decrease in serum thyroid hormone levels.