title: Levothyroxine dosage is associated with stability of thyroid-stimulating hormone values.
abstract: OBJECTIVE
  In patients treated for hypothyroidism, the usual practice is to monitor thyroid-stimulating hormone values yearly once a therapeutic dosage of levothyroxine is determined. This study investigates whether there are any clinical predictors that could identify a subset of patients who might be monitored safely on a less frequent basis.
  
  
  METHODS
  With the use of a retrospective study design, 715 patients treated for hypothyroidism who had a normal (ie, therapeutic) thyroid-stimulating hormone value in 2006 while taking levothyroxine were identified. All thyroid-stimulating hormone values were then obtained through December 31, 2012. By using a Cox proportional hazard model, gender, age, body mass index, history of chronic autoimmune thyroiditis, initial thyroid-stimulating hormone level, and levothyroxine dose were analyzed for time to first abnormal thyroid-stimulating hormone value.
  
  
  RESULTS
  Age, gender, history of chronic autoimmune thyroiditis, and body mass index at the time of initial normal thyroid-stimulating hormone were not associated significantly with time to abnormal thyroid-stimulating hormone value. Levothyroxine dose >125 μg/day had an increased hazard ratio of 2.4 (95% confidence interval, 1.7-3.4; P < .0001) for time to first follow-up abnormal thyroid-stimulating hormone value, but dosages less than that did not increase the hazard ratio. One year after the initial normal thyroid-stimulating hormone value, 91.1% of patients taking ≤ 125 μg/day had a continued normal thyroid-stimulating hormone, whereas only 73.3% of patients taking >125 μg/day did. Transformed thyroid-stimulating hormone value (which represents a measure of how far the initial thyroid-stimulating hormone was from the midpoint of the normal range) also had an increased hazard ratio of 1.14 (95% confidence interval, 1.1-1.2; P < .0001) for time to first abnormal thyroid-stimulating hormone value.
  
  
  CONCLUSIONS
  For patients receiving ≤ 125 μg/day of levothyroxine, we propose that a testing interval up to 2 years may be acceptable if their thyroid-stimulating hormone is well within the normal range.
authors: Department of Family Medicine, Mayo Clinic, Rochester, Minn. Electronic address: Pecina.jennifer@mayo.edu.
authors: Jennifer
authors: J
authors: Pecina
authors: Department of Family Medicine, Mayo Clinic, Rochester, Minn.
authors: Gregory M
authors: GM
authors: Garrison
authors: Department of Family Medicine, Mayo Clinic, Rochester, Minn.
authors: Matthew E
authors: ME
authors: Bernard
mesh_terms: D000328:Adult; D000368:Aged; D015415:Biomarkers / Q000097:blood; D005260:Female; D006801:Humans; D007037:Hypothyroidism / Q000097:blood* / Q000188:drug therapy*; D008297:Male; D008875:Middle Aged; D016017:Odds Ratio; D012189:Retrospective Studies; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000008:administration & dosage*; D013997:Time Factors
chemical_list: D015415:Biomarkers; D013972:Thyrotropin; D013974:Thyroxine
keywords: Hypothyroidism; Hypothyroidism treatment; Levothyroxine dosage; Thyroid-stimulating hormone