To analyze the long-term impact of birth weight (BW) on thyroid function in genetically identical twins with intra-twin BW differences from birth to adolescence.

In total, 52 monozygotic twin pairs underwent at least one analysis of thyroid function at mean ages of 10.1 years (27 pairs), 15.1 years (35 pairs), and 17.4 years (36 pairs); 18 pairs donated blood at all time points. BW difference of <1 SDS was defined as concordant, BW difference ≥1 SDS as discordant.

In concordant twins, no significant differences were observed. In the discordant group, smaller twins had higher mean thyroid-stimulating hormone (TSH) than their larger co-twins at 10.1 years (3.6 vs 2.5 μU/mL; P = .04) and 15.1 years (2.6 vs 2.2 μU/mL; P = .08). Smaller twins showed lower mean thyroxine than larger co-twins at 10.1 years (7.8 vs 8.2 μg/dL P = .05) and 17.4 years (7.7 vs 8.4 μg/dL; P = .03), and a tendency at 15.1 years (6.9 vs 7.4 μg/dL; P = .09). Calculation of TSH-thyroxine ratio revealed significant differences in the discordant group, with greater ratios in the smaller twin at 10.1 years (0.5 vs 0.3; P = .006) and 15.1 years (0.4 vs 0.3; P = .04).

In this group of monozygotic twins with intra-twin BW differences, BW seemed to exert a long-lasting impact on thyroid function. This may be due to a delay in hypothalamic−pituitary−thyroid axis maturation, with TSH resistance during childhood and early adolescence in children with low BW.