Abstract
Purpose: To investigate how stable the concentrations of thyroid hormones are during a 10 year period in healthy women by estimation of within- and between-subject biological variation.
Material: 117 women (48 to 60 years) without thyroid disease treated with (n=28) or without (n=89) estrogens, and participating in the Danish Osteoporosis Prevention Study, were investigated by blood sampling 6 times during a 10 year period: 0, ½, 1, 2, 5 and 10 year. A questionnaire was filled in and a clinical investigation was performed. Medication including estrogens as well as thyroid medication was accounted for and confirmed by our pharmacy database.
Methods: TSH, free thyroxine (FT4) and triiodothyronine (FT3) were determined on AutoDELFIA (Perkin Elmer/Wallac). Biological variation was estimated from nested analysis of variance. Results The estimated CV within and CV between for TSH and free thyroid hormones during 10 years were for the two groups with and without hormone replacement therapy (HRT): TSH: CV-within = 42.0% and CV-between = 59.1% without HRT and CV-within = 37.0% and CV-between = 31.9% with HRT Free T4: CV-within = 7.8% and CV-between = 10.9 % without HRT and CV-within = 8.8% and CV-between = 10.8% with HRT Free T3: CV-within = 6.8% and CV-between = 8.0% without HRT and CV-within = 8.6% and CV-between = 6.1% with HRT Discussion The biological variation for TSH, but not for free T4 and free T3, is larger than that previously published in short term studies. CV-values for the free thyroid hormones are not significantly influenced by estrogen use.
Conclusions: Serum free T4 and free T3 remain more constant than hitherto thought over a long time period. This facilitates the possibility of detecting early stages of thyroid diseases.