Thyrotropin and thyroid antibodies as predictors of hypothyroidism: a 13-year, longitudinal study of a community-based cohort using current immunoassay techniques.
BACKGROUND
Longitudinal studies of risk factors for hypothyroidism are required to inform debate regarding the TSH reference range. There are limited longitudinal data on the predictive value of thyroid antibodies measured by automated immunoassay (as opposed to semiquantitative methods).


METHODS
We measured TSH, free T(4), thyroid peroxidase antibodies (TPOAbs), and thyroglobulin antibodies (TgAbs) using the Immulite platform on sera from 1184 participants in the 1981 and 1994 Busselton Health Surveys. Outcome measures at follow-up were hypothyroidism, defined as TSH greater than 4.0 mU/liter or on thyroxine treatment; and overt hypothyroidism, defined as TSH above 10.0 mU/liter or on thyroxine treatment. Receiver-operator characteristic analysis was used to determine optimal cutoffs for baseline TSH, TPOAbs, and TgAbs as predictors of hypothyroidism.


RESULTS
At 13 yr follow-up, 110 subjects (84 women) had hypothyroidism, of whom 42 (38 women) had overt hypothyroidism. Optimal cutoffs for predicting hypothyroidism were baseline TSH above 2.5 mU/liter, TPOAbs above 29 kIU/liter, and TgAbs above 22 kIU/liter, compared with reference range upper limits of 4.0 mU/liter, 35 kIU/liter, and 55 kIU/liter, respectively. In women with positive thyroid antibodies (TPOAbs or TgAbs), the prevalence of hypothyroidism at follow-up (with 95% confidence intervals) was 12.0% (3.0-21.0%) when baseline TSH was 2.5 mU/liter or less, 55.2% (37.1-73.3%) for TSH between 2.5 and 4.0 mU/liter, and 85.7% (74.1-97.3%) for TSH above 4.0 mU/liter.


CONCLUSIONS
The use of TSH cutoffs of 2.5 and 4.0 mU/liter, combined with thyroid antibodies, provides a clinically useful estimate of the long-term risk of hypothyroidism.
Walsh
John P
JP
Department of Endocrinology and Diabetes, Sir Charles Gairdner Hospital, Nedlands, Western Australia 6009. john.walsh@health.wa.gov.au
Bremner
Alexandra P
AP
Feddema
Peter
P
Leedman
Peter J
PJ
Brown
Suzanne J
SJ
O'Leary
Peter
P
D000328:Adult; D000367:Age Factors; D000368:Aged; D001315:Australia; D001323:Autoantibodies / Q000097:blood*; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007037:Hypothyroidism / Q000097:blood* / Q000175:diagnosis* / Q000453:epidemiology; D007118:Immunoassay; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D018579:Patient Selection; D011237:Predictive Value of Tests; D015995:Prevalence; D011379:Prognosis; D012372:ROC Curve; D012016:Reference Values; D012044:Regression Analysis; D012307:Risk Factors; D012680:Sensitivity and Specificity; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000097:blood
D001323:Autoantibodies; D013972:Thyrotropin; D013974:Thyroxine