Interesting paper which shows that (in the dog) elevated TSH stimulates the deiodinase enzyme that converts rT3 into T2. This means that if someone for example takes so much excess T4 as to suppress TSH and thereby raise rT3 levels as a "drain" to remove the T4 excess, this inhibits the removal of rT3 and potentiates possible interference with T3 activity. Another reason to be cautious about T4 monotherapy if a patient to get sufficient FT3 has to take more because of poor T4-T3 conversion. It suggests that the way out of this dilemma is to add T3 to avoid the rT3 buildup from excess T4 and TSH suppression.
Thyrotropin-Mediated Induction of Thyroidal Iodothyronine Monodeiodinases in the Dog
Nuclear Medicine Service, Veterans Administration Medical Center, Long Beach, and University of California, Irvine, California 90822
Endocrinology Vol. 112, No. 2
, 417-422 (1983)