The possible reason for serum 3,3'5'-(reverse) triiodothyronine increase in old people.

Healthy subjects aged 70-90 and 20-40 years were investigated for the level of reverse triiodothyronine (rT3). The mean level in 57 old subjects was higher and the triiodothyronine (T3) level lower than in 30 young persons. No negative correlation was observed. A serum T3 elevation caused by liothyronine administration (5-15 micrograms/day for two weeks) was not followed by a change of the mean rT3 level in 12 old subjects. After a single intramuscular thyrotropin (TSH) injection, the rT3 elevation lasted 24h longer in 19 old persons than in the 9 control ones, though the early rT3 response was similar in both groups. Propranolol administration (40 mg t.i.d. for a week) caused a similar rT3 elevation in old persons (n = 18) as in 12 young ones. From the indirect data it is concluded that the serum rT3 elevation in old age is a result of faster peripheral generation and not of increased production in the thyroid gland. The data do not indicate any change of rT3 metabolism. T4-T3 and T4-rT3 conversion are provoked by different enzymes. The elevation of rT3 might be a cause of the observed decrease in peripheral T3 generation in old subjects, acting by an inhibition of the T4-T3 conversion.