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 hypothesised 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 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 response of plasma THs to short-term fasting. In general, plasma total T₃ and total T₃/T₄ ratio correlated negatively, while 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 fractional growth rates 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, while catch-up growth relates to increased abundance of the more bioactive forms of TH. Finally, greater soft tissue growth occurs in PR compared to 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.