Low prevalence of hypothyroidism among black and Mulatto people in a population-based study of Brazilian women

Objective African-Americans have been shown to have low prevalence of hypothyroidism. Brazil has a high ethnic admixture allowing further exploration into whether environmental factors can explain the ethnic differences. Design A survey, representative of the population of Rio de Janeiro, a large metropolitan city in Brazil. Factors studied included race, parity, income, schooling, and smoking. Population The survey was carried out in Rio de Janeiro whereby households (1500) were selected using three-stage probability sampling. A total of 1298 (86.5%) women participated in the survey (non-response: 13.5%). Measurements TSH from blood drawn at the households. Anti-thyroperoxidase (anti-TPO) antibodies and free T4 were also measured. Results Overall prevalence of hypothyroidism (TSH > 4 mUI/ml or taking medication) was 12.3%. Prevalence was 6.9% in black people, 8.8% in Mulatto people, and 16.7% among white people. The mean serum TSH of the population was 2.65 (95% confidence interval 2.33-2.97). The TSH distribution of black and Mulatto people was shifted to the left compared to white people. After adjustment for age, income, smoking and presence of anti-TPO, Black and Mulatto people were still protected from hypothyroidism. The adjusted odds ratio for black compared to white people was 0.45 (95% CI 0.30-0.68) and for Mulatto people was 0.34 (95% CI 0.18-0.63). Serum TSH levels were significantly lower in smokers than in non-smokers, but there was no association between number of cigarettes smoked and serum TSH level. Conclusions This is the first time it has been demonstrated that Mulatto people have a prevalence of hypothyroidism which lies between that of white and black people, independent of the prevalence of anti-TPO and smoking.