Abstract
Variability in thyroid function in pregnant women is the result of 2 main determinants, each accounting for approximately half of it. The first is the genetically determined part of which the knowledge increases fast, but most remains to be discovered. The second determinant is caused by an ensemble of variables of which thyroid autoimmunity is the best known, but also by others such as parity, smoking, age, and BMI. More recently, new candidate variables have been proposed, such as iron, endocrine disruptors, and the ethnicity of the pregnant women. In the future, the diagnosis and treatment of thyroid (dys)function may be optimized by the use of each individual's pituitary-thyroid set point, corrected with a factor taking into account the impact of nongenetically determined variables.