Natural Course of Euthyroidism and Clues for Early Diagnosis of Thyroid Dysfunction: Tehran Thyroid Study

Objective: Considering the limited data available on the natural course of euthyroidism, this study was designed to evaluate the progression in time from euthyroidism to subclinical or overt hypo- or hyperthyroidism. Methods: This study was conducted within the framework of the Tehran Thyroid Cohort Study, in which 5783 individuals aged 40.4 ± 0.2 years were followed for six years. The overall loss to follow-up rate was 8.3%. After applying exclusion criteria, data of 4204 euthyroid subjects remained for analysis of a six-year natural course analysis. Thyroid function tests, clinical characteristics, and metabolic characteristics were assessed at baseline and every three years. Results: The annual incidence rates [confidence intervals (CI)] of subclinical and overt hypothyroidism were 7.62 [CI 7.39–7.85) and 2.0 [CI 1.94–2.06] per 1000 persons, respectively. For thyroid hyperfunction, the annual incidence rates of subclinical and overt hyperthyroidism were 0.92 [0.90–0.95) and 0.68 [0.66–0.70) per 1000 persons, respectively. Euthyroid persistency was 93.24% during 6 years. Predictive factors for conversion to thyroid dysfunction were thyrotropin, free thyroxine and thyroid peroxidase antibody levels, sex, and smoking. Criteria for early diagnosis of hypothyroidism (i.e., sensitivity of 94% and specificity of 82%, p