In 2010, 152 monozygotic twins with extremely different birth weights were identified from the Danish Twin Registry. When participants were analysed as singletons, birth weight was inversely associated with serum levels of free T4 and T3, but not with serum TSH levels. When participants were analysed as twin pairs, serum levels of all three markers were similar between twins of low and high birth weight. The authors conclude that birth weight has no effect on the pituitary–thyroid axis in adults.