Possible role of genetic factors in thyroid growth rate and in the assessment of upper limit of normal thyroid volume in iodine-replete adolescents

The objective of this study was to answer the question whether thyroid volu me in adolescent siblings of similar age and a life-long sufficient iodine intake is uniform. If different, it would indicate that genetic or environm ental factors unrelated to iodine intake can influence thyroid growth. We m easured thyroid volume by ultrasound in: (1) 251 sibling pairs (SP) and 19 sibling triads 10 to 18 years of age. The age range of each SP was less tha n 24 months and of each triad less than 42 months; (2) 28 monozygotic and 1 3 dizygotic sets of twins 7 to 18 years of age. The sibling pairs were retr ospectively divided into 3 groups irrespective of age (thyroid volume as me ans +/- S.E. mL/m(2)). Group 1: 159 pairs with low thyroid volume in both s iblings; mean thyroid volume of each pair less than 5.00 mL/m(2) (3.36 +/- 0.05, median 4.08, range 2.07-4.98); group 2: 69 pairs with high thyroid vo lume in both siblings; mean thyroid volume greater than 5.00 mL/m(2) (5.85 +/- 0.12, median 5.57, range 5.03-11.02); group 3: 23 pairs with low thyroi d volume in 1 sibling (3.53 +/- 0.15, median 3.53, range 1.71-4.91) and hig h thyroid volume in another (7.36 +/- 0.23, median 7.18, range 5.36-10.30). The majority of triads, monozygotic, and dizygotic twins resembled group 1 , a few resembled group 2, and only 3 triads and 1 set of dizygotic twins r esembled group 3. Among monozygotic twins, there was no pair with a strikin gly discordant thyroid volume and only 1 such pair was found among dizygoti c twins. In monozygotic twins, the thyroid volume was almost identical (mea n difference 0.34 +/- 0.06 mL/m(2)) and significantly less (P < 0.012) than in dizygotic twins (0.9 +/- 0.25 mL/m(2)). Among 502 children of 251 sibli ng pairs the frequency of high thyroid volume (>5.00 mL/m(2)) was greater i n girls (103/273, 36.3%, p < 0.01) than in boys (49/223, 22.0%). The same w as true for the frequency of hypoechogenicity (42/279 or 15.0% in girls vs. 12/223 or 5.4% in boys; p < 0.01). The frequency of hypoechogenicity in bo th sexes of the combined groups 2 and 3 (40/186, 21.5%) was higher (p < 0.0 01) than in group 1 (14/316, 4.4%). All siblings examined lived in a common household with their parents, eating the same daily meals at home and scho ol. Our results suggest that the observed differences in thyroid volume of siblings were not related to iodine intake, but to other factors, eg, genet ic and environmental. It is not clear whether the children with high thyroi d volume and increased frequency of hypoechogenicity should be included int o the recently recommended range of normal thyroid volume for adolescents.