Underestimation of thyroid dysfunction risk due to regression dilution bias in a long-term follow-up: Tehran Thyroid Study (TTS)

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 < 0.0001) were obtained based on baseline and three-year follow-ups of thyroid function tests and thyroid peroxidase antibody. Early diagnosis of hypothyroidism was significantly associated with impaired glucose tolerance (relative risk with 3.03 [CI 1.36-6.75]; p = 0.007), high cholesterol (relative risk 2.46 [CI 1.45-4.18]; p = 0.001), obesity (relative risk 2.92 [CI 1.64-5.2]; p < 0.001), and hypertension (relative risk 1.68 [CI 1.53-1.84]; p < 0.04).

CONCLUSION

This study shows that after a six-year follow-up in an iodine sufficient area, 6.7% of euthyroid subjects were found to progress to thyroid dysfunction, in particular subclinical hypothyroidism.

Underestimating the Effect of Lipids on Cardiovascular Events: Regression Dilution Bias in the Population-Based Cohort of Tehran Lipid and Glucose Study

BACKGROUND

Random errors in the measurement of risk factors lead to bias in the exposure-disease association.

OBJECTIVES

This study aimed to examine the extent of underestimation in the association of total cholesterol (TC), high density lipoprotein cholesterol (HDL-C) and triglyceride (TG) with cardiovascular disease (CVDs) in the Tehran Lipid and Glucose Study.

PATIENTS AND METHODS

Of 6327 eligible people in the original cohort followed for about 10 years to detect CVD events, 3063 (1224 men and 1839 women) had replicate measurements for blood lipids. Two regression dilution ratios were calculated by nonparametric and parametric methods, using replicate data from reexaminations three and six years after baseline. Adjusted and unadjusted hazard ratios (HR) were corrected for regression dilution bias.

RESULTS

By parametric method, based on reexamination three years after baseline, the strength of real association of usual levels of TC, TG and HDL-C with cardiovascular disease, considering β coefficients of related models, were underestimated about 42%, 51% and 81% in men and 40%, 43% and 62% in women, respectively. Underestimations were relatively independent of age, sex and body mass index. Correction for regression dilution bias led to more than 60% increase in estimated HR for TC adjusted for confounders.

CONCLUSIONS

Using baseline measurements of lipids led to considerable underestimation in the association of these factors with CVD outcome in TLGS. The underestimation increased with time interval between baseline and follow-up measurements for TC and TG. TC had more attenuation on estimated HR due to stronger relation with CVD risk.