title: Circadian and Circannual Rhythms in Thyroid Hormones: Determining the TSH and Free T4 Reference Intervals Based Upon Time of Day, Age, and Sex.
abstract: BACKGROUND
  Establishing the reference interval for thyrotropin (TSH) and free thyroxine (T4) is clinically important because a number of disease states have been linked to alterations in TSH and free T4 concentrations that are within the 95% confidence interval for normal thyroid hormone values. Age, sex, time of day, and ethnicity are known to affect circulating levels of TSH and free T4 but have not been used to establish reference intervals. The purpose of this study was to define the reference interval for TSH and free T4 taking into account age, sex, ethnicity, and circadian and circannual variability.
  
  
  METHODS
  We performed a retrospective analysis of 465,593 TSH and 112,994 free T4 measurements from subjects ages 1-104 years with no thyroid disease using a single TSH and free T4 immunoassay method. Boundaries for the central 95% of patient values, taking into account hour of day, day of year, sex, and age were calculated.
  
  
  RESULTS
  Females had significantly higher TSH and free T4 levels than males; the magnitude of these differences did not exceed 0.1 mIU/L or 0.1 ng/dL respectively. Although the 2.5% TSH reference interval remains constant through the day, date, and age ranges, the upper limit (97.5%) of the TSH reference interval increases from 6.45 to 7.55 mIU/L with age, due primarily to a progressive increase in the amplitude of the nocturnal TSH surge. Additionally, significant ethnic differences in TSH circadian periodicity occur between African American, Pacific Island, and Caucasian populations.
  
  
  CONCLUSIONS
  The reference interval for TSH varies significantly by age, sex, hour of day, and ethnicity. Time of year does not affect the TSH reference interval, and age, sex, hour of day and time of year do not affect the free T4 reference interval.
authors.affiliation: 1 Department of Medicine, Intermountain Central Laboratory , Murray, Utah.
authors.forename: Joel
authors.initials: J
authors.lastname: Ehrenkranz
authors.affiliation: 2 Central Laboratory, Intermountain Healthcare , Murray, Utah.
authors.forename: Phillip R
authors.initials: PR
authors.lastname: Bach
authors.affiliation: 3 Statistical Data Center, Intermountain Healthcare Research , Salt Lake City, Utah.
authors.forename: Gregory L
authors.initials: GL
authors.lastname: Snow
authors.affiliation: 4 Department of Medicine, McKee-Dee Hospital , Ogden, Utah.
authors.forename: Alison
authors.initials: A
authors.lastname: Schneider
authors.affiliation: 5 Intermountain Healthcare , Salt Lake City, Utah.
authors.forename: Jo Lynn
authors.initials: JL
authors.lastname: Lee
authors.affiliation: 2 Central Laboratory, Intermountain Healthcare , Murray, Utah.
authors.forename: Sarah
authors.initials: S
authors.lastname: Ilstrup
authors.affiliation: 2 Central Laboratory, Intermountain Healthcare , Murray, Utah.
authors.forename: Sterling T
authors.initials: ST
authors.lastname: Bennett
authors.affiliation: 6 Department of Clinical and Experimental Medicine, Policlinico Universitario di Messina , Messina, Italy .
authors.forename: Salvatore
authors.initials: S
authors.lastname: Benvenga
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D002940:Circadian Rhythm; D016208:Databases, Factual; D005260:Female; D006801:Humans; D007118:Immunoassay; D007223:Infant; D008297:Male; D008875:Middle Aged; D010045:Outpatients; D012016:Reference Values; D012189:Retrospective Studies; D013959:Thyroid Diseases / Q000097:blood; D013960:Thyroid Function Tests; D013963:Thyroid Hormones / Q000097:blood*; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000097:blood*; D013997:Time Factors; D014284:Triiodothyronine / Q000097:blood; D055815:Young Adult
chemical_list: D013963:Thyroid Hormones; D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine