Effects of total fasting in obese women. IV. Response of serum triiodothyronine (T3) and reverse triiodothyronine (rT3) to administered T3.

Serum levels of T4, T3, rT3, RT3U and TSH were estimated in 12 obese women in the course of a 14-day fasting. Seven of these patients were treated with T3 in a daily dose of 60--80 micrograms. Fasting led to a small increase of serum T4, while fasting combined with T3 administration was accompanied by a small decrease of serum T4. Serum T3 decreased in the course of the first 2 days of fasting to 40--50% of initial values and remained at this low level up to the 15th day of fasting. In the T3 treated group a prompt increase of serum T3 was recorded (+80%), followed by a steady decrease, reaching the control values on day 15, in spite of a continuous T3 administration. Serum rT3 in the untreated fasting group steadily increased up to the 12th day (+43%) and then dropped below the control value (-28%). In the treated group after a non-significant increase (+22%), a decrease of serum rT3 was also observed (-42%). Fasting was accompanied by a tendency to increased serum RT3U values, in the T3 treated group no change occurred. TSH in the untreated fasting women remained practically unchanged while T3 administration was accompanied by a tendency to a transient decrease. The heart rate showed a non-significant tendency to increase in the T3 treated group and the last week of fasting was also accompanied in several patients by a larger weight loss. These results suggest that in addition to an enhanced synthesis of rT3 to the detriment of a decreased production of T3 from T4, an increased catabolism of administered T3 and of endogenous rT3 is involved in a strict calorie restriction. An abortive T3-TRH-TSH feed-back may also operate in these conditions.