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The relationships among avian uncoupling protein (avUCP) mRNA expression, heat production, and thyroid hormone metabolism were investigated in 7–14-day-old broiler chicks (Gallus gallus) exposed to a low temperature (cold-exposed chicks, CE) or a thermoneutral temperature (TN). After 7 days of exposure, CE chicks exhibited higher heat production (+83%, P<0.01), avUCP mRNA expression (+20%, P<0.01), and circulating triiodothyronine (T₃) levels (+104%, P=0.07) for non-statistically different body weights and feed intake between 3 and 7 days of exposure as compared to TN chicks. Plasma thyroxine (T₄) concentration was clearly decreased in CE chicks (−33%, P=0.06). The lower hepatic inner-ring deiodination activity (−47%) and the higher renal outer-ring deiodination activity (+75%) measured in CE compared to TN chicks could partly account for their higher plasma T₃ concentrations. This study …