Placental restriction alters circulating thyroid hormone in the young lamb postnatally.

Intrauterine growth restriction (IUGR) is associated with accelerated growth and increased adiposity in early life due to unknown mechanisms, which could include increased thyroid hormone (TH) action. We hypothesized that placental restriction (PR) of fetal growth would increase circulating TH concentrations and alter their response to fasting, and that these would relate to growth and body composition in the young lamb. PR reduced size at birth, increased fractional growth rates (FGRs) of soft and skeletal tissues up to 30 days of age, and slowed the ontogenic decrease in plasma total T3 and plasma total T3/T4. PR did not alter the abundance of plasma THs after short-term fasting. In general, plasma total T3 and total T3/T4 ratio correlated negatively, whereas plasma total T4 correlated positively with size at birth. Absolute growth rates of weight and crown-rump length correlated positively with plasma total T3 and total T4 between days 15 and 35. Current FGRs for weight and metatarsal length correlated positively with plasma total T3 between days 20 and 35. In conclusion, PR and small size at birth reduce plasma total T4 and increase plasma total T3 postnatally, whereas catch-up growth relates to increased abundance of the more bioactive forms of TH. Finally, greater soft tissue growth occurs in PR compared with control lambs at the same circulating TH concentrations. This suggests that PR and small size at birth may increase activation of T4 to T3 and sensitivity of soft tissues to TH, which may contribute to catch-up growth following IUGR.