Reference interval of thyroid stimulating hormone and free thyroxine in a reference population over 60 years old and in very old subjects (over 80 years): comparison to young subjects.

BACKGROUND: Studies based on laboratory data about thyroid stimulating hormone (TSH) and free thyroxine (FT4) reference interval (RI) show conflicting results regarding the importance of using specific values by age groups with advancing age. Retrospective laboratory data or non-specific criteria in the selection of subjects to be studied may be factors leading to no clear conclusions. The aim of this study is to test the hypothesis that TSH and FT4 have specific RI for subjects over 60 to 80 years.

METHODS: We evaluated prospectively 1200 subjects of both sexes stratified by age groups, initially submitted to a questionnaire to do the first selection to exclude those with factors that could interfere in TSH or FT4 levels. Then, we excluded those subjects with goiter or other abnormalities on physical examination, positive thyroid peroxidase antibodies (TPOAb), thyroglobulin antibodies (TGAb), and other laboratory abnormalities.

RESULTS: TSH increased with age in the whole group. There was no statistical difference in the analysis of these independent subgroups: 20-49 versus 50-59 years old (p > 0.05), and 60-69 versus 70-79 years old (p > 0.05). Consequently, we achieved different TSH RI for the three major age groups, 20 to 59 years old: 0.4 - 4.3 mU/L, 60 to 79 years old: 0.4 - 5.8 mU/L and 80 years or more: 0.4 - 6.7 mU/L. Conversely, FT4 progressively decreases = significantly with age, but the independent comparison test between the sub-groups showed that after age 60 the same RI was obtained (0.7 - 1.7 ng/dL) although the minimum value was smaller than that defined by manufacturer. In the comparison between TSH data obtained by this study and those defined by the manufacturer (without segmentation by age) 6.5% of subjects between 60 and 79 years and 12.5% with 80 years or more would have a misdiagnosis of elevated TSH.

CONCLUSIONS: TSH normal reference range increases with age, justifying the use of different RI in subjects 60 years old and over, while FT4 decreases with age. Using specific-age RI, a significant percentage of elderly will not be misdiagnosed as having subclinical hipothyroidism.