title: The thyroid axis 'setpoints' are significantly altered after long-term suppressive LT4 therapy.
abstract: 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.
authors.affiliation: Department of Nuclear Medicine, University of Würzburg, Würzburg, Germany.
authors.forename: F A
authors.initials: FA
authors.lastname: Verburg
authors.affiliation: Comprehensive Cancer Center, University of Würzburg, Würzburg, Germany.
authors.forename: U
authors.initials: U
authors.lastname: Mäder
authors.affiliation: Department of Nuclear Medicine, University of Würzburg, Würzburg, Germany.
authors.forename: I
authors.initials: I
authors.lastname: Grelle
authors.affiliation: Department of Internal Medicine, Erasmus University Medical Center, Rotterdam, The Netherlands.
authors.forename: T J
authors.initials: TJ
authors.lastname: Visser
authors.affiliation: Department of Internal Medicine, Erasmus University Medical Center, Rotterdam, The Netherlands.
authors.forename: R P
authors.initials: RP
authors.lastname: Peeters
authors.affiliation: Department of Internal Medicine, University Medical Center St. Radboud, Nijmegen, The Netherlands.
authors.forename: J W A
authors.initials: JW
authors.lastname: Smit
authors.affiliation: Department of Nuclear Medicine, University of Würzburg, Würzburg, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Reiners
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D001835:Body Weight / Q000187:drug effects; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D013961:Thyroid Gland / Q000187:drug effects / Q000378:metabolism*; D013972:Thyrotropin / Q000378:metabolism; D013974:Thyroxine / Q000378:metabolism / Q000494:pharmacology*; D014284:Triiodothyronine / Q000378:metabolism; D055815:Young Adult
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine