Timing of levothyroxine administration affects serum thyrotropin concentration.
BACKGROUND
Patients treated with levothyroxine typically ingest it in a fasting state to prevent food impairing its absorption. The serum thyrotropin concentration is the therapeutic index of levothyroxine action.


OBJECTIVE
The study objective was to determine the effect of the timing of levothyroxine administration in relationship to food on serum thyrotropin levels.


METHODS
Participants were randomized to one of six sequences, each consisting of three 8-wk regimens in a three-period crossover design. These regimens were in a fasting state, at bedtime, and with breakfast. The concentrations of TSH, free T(4), and total T(3) during each of the three timing regimens were documented. The primary outcome was the difference between serum TSH concentrations under fasting conditions compared with concentrations during the other 8-wk regimens.


METHODS
The study was conducted in an academic medical center.


METHODS
Study participants were receiving levothyroxine for treatment of hypothyroidism or thyroid cancer.


RESULTS
Sixty-five patients completed the study. The mean thyrotropin concentration was 1.06 +/- 1.23 mIU/liter when levothyroxine was administered in the fasting state. When levothyroxine was taken with breakfast, the serum thyrotropin concentration was significantly higher (2.93 +/- 3.29 mIU/liter). When levothyroxine was taken at bedtime, the serum TSH concentration was also significantly higher (2.19 +/- 2.66 mIU/liter).


CONCLUSIONS
Nonfasting regimens of levothyroxine administration are associated with higher and more variable serum TSH concentrations. If a specific serum TSH goal is desired, thereby avoiding iatrogenic subclinical thyroid disease, then fasting ingestion of levothyroxine ensures that TSH concentrations remain within the narrowest target range.
Bach-Huynh
Thien-Giang
TG
Division of Endocrinology, Georgetown University Medical Center, Washington, DC 20007, USA.
Nayak
Bindu
B
Loh
Jennifer
J
Soldin
Steven
S
Jonklaas
Jacqueline
J
D000284:Administration, Oral; D000328:Adult; D000368:Aged; D018592:Cross-Over Studies; D004334:Drug Administration Schedule; D005215:Fasting; D005260:Female; D020249:Hormone Replacement Therapy; D006801:Humans; D007037:Hypothyroidism / Q000188:drug therapy; D008297:Male; D008875:Middle Aged; D013963:Thyroid Hormones / Q000097:blood; D013964:Thyroid Neoplasms / Q000601:surgery; D013972:Thyrotropin / Q000097:blood* / Q000187:drug effects; D013974:Thyroxine / Q000008:administration & dosage*; D013997:Time Factors
D013963:Thyroid Hormones; D013972:Thyrotropin; D013974:Thyroxine