Thyroid Research Thyroid Research Archive Thyroid Hormone Secretion

Genetic factors are more important than environmental factors in determining pituitary–thyroid function in normal subjects

(July 2004)

The background of the study. Serum thyroid hormone and thyrotropin (TSH) concentrations vary less in an individual normal subject than among groups of subjects, suggesting that each subject has a unique setting for TSH and, therefore, thyroid secretion. In this study pituitary–thyroid function was assessed in monozygotic and dizygotic twin pairs in an attempt to determine the genetic influence on this setting.

How the study was done. Serum TSH, free thyroxine (T4), and triiodothyronine (T3) were measured in 690 Danish twin pairs (692 women and 688 men), including 284 monozygotic twin pairs, 286 same-sex dizygotic twin pairs, and 120 opposite-sex twin pairs. The mean age of all three types of twin pairs was 37 years.

The results of the study. The mean serum TSH concentrations were 1.8 mU/L in the women and 1.6 mU/L in the men. Serum TSH and free T4 or free T3 concentrations were not linearly related to each other in any of the three groups.

Among the monozygotic twin pairs, the serum TSH, free T4, and free T3 concentrations in twins 1 and 2 were correlated. For the same-sex dizygotic twin pairs, the values were less closely correlated. Based on quantitative genetic modeling, genetic components were estimated to account for 64 to 65 percent of the variation in serum TSH, free T4, and free T3 concentrations and environmental effects for 35 to 36 percent.

The conclusions of the study. Serum TSH and thyroid hormone concentrations are more closely correlated with each other in monozygotic than dizygotic twins, indicating that genetic factors are important determinants of variations in pituitary–thyroid function in normal subjects.

The original article. Hansen PS, Brix TH, Sorensen TI, Kyvik KO, Hegedus L. Major genetic influence on the regulation of the pituitary-thyroid axis: a study of healthy Danish twins. J Clin Endocrinol Metab 2004;89:1181-7.