The aim of the study was to investigate the changes in the thyroid axis setpoint after long-term suppressive
levothyroxine therapy for differentiated
thyroid carcinoma and the resulting changes in
levothyroxine requirement. Ninety-nine differentiated
thyroid cancer patients were reviewed. All patients had at least one known TSH-level≥0.01 mU/l (lower detection limit) and <1.0 mU/l within 2 years of initial treatment (time 1) and had at least one TSH-value≥0.01 mU/l and <1.0 mU/l after continuous LT4
therapy for a minimum of 5 years (time 2).At time 2 the mean LT4 dosage/kg
body weight, TSH, FT3, and FT4 levels were significantly lower than at time 1, while
body weight was higher. At time 2, the FT3/FT4 ratio rate had dropped significantly (p<0.001). At time 1, patients would require 2.96 μg/kg
body weight to reach total TSH suppression. The dose of
levothyroxine/kg required for suppression can be lowered by about 0.05 μg/kg
body weight for each year of suppressive
therapy. After a median of 12.7 years of continuous suppressive
levothyroxine therapy, patients would require 2.25 μg/kg
body weight (-23.5%) to reach total TSH-suppression. At least part of this reduction was independent of aging. As a result of changes in
thyroid hormone metabolism and thyroid axis setpoint, long-term TSH-suppressive
therapy contributes to a reduction in the dosage of
levothyroxine per kilogram
body weight required for full TSH suppression over time.