The relationship between heat production and concentrations of plasma thyroid hormones in the domestic hen

Abstract

A low-amplitude daily rhythm of heat production was seen in sexually immature hens exposed to 14 hr light/day at 30° with the lowest values occurring during the dark period. Thyroidectomy depressed heat production during the light and dark periods. Fasting depressed the level of the rhythm of heat production in intact hens and depressed it further in the thyroidectomized hens. The amplitude of the daily rhythm of heat production in laying hens exposed to 14 hr light/day and held at 20° was greater than that in the same birds held at 32°. The increase in temperature caused a reduction in heat production during the light period but not during the dark period. Fasting lowered heat production during the light and dark periods when the hens were held at 20° and during the dark period when they were held at 32°. The increase in temperature caused a reduction in the concentration of plasma triiodothyronine (T 3) but did not alter the concentration of plasma thyroxine (T 4). In fed hens held at 20 and 32°, the concentration of plasma T 3 increased during the light period and fell during the dark period: the converse changes were observed in the concentrations of plasma T 4. In fasted hens held at 20 and 32°, the concentration of plasma T 3 was uniformly depressed during the light and dark periods: the concentration of plasma T 4 was increased during the light and dark periods but tended to fall after the birds had been fasting for 48 hr. It was concluded that in the hen (1) heat production is related to the concentration of plasma T 3 and not to that of plasma T 4, and (2) daily rhythms in concentrations of plasma thyroid hormones are controlled by the feeding pattern.