Response of the normal and thyroidectomized cat to serve cold

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Abstract
Anesthetized, artificially respired normal and thyroidectomized cats were subjected to –20°C air temperature following 1 hour equilibration at room temperature. Blood pressure, heart rate, I131 albumin blood volume, Na24 dilution volume, hematocrit, and rectal, subdermal, intra-abdominal and esophageal temperatures were measured continuously until arterial blood pressure was absent. At room air temperature thyroidectomized animals had lower body temperatures, blood pressure, heart rate and plasma volumes, and greater Na24 dilution volumes and plasma hematocrits. Both groups exhibited similar responses to sudden and severe cold: immediate increases in heart rate and blood pressure were followed by a sharp and continuous decline. Simultaneously occurred an abrupt decrease in plasma hematocrit, a sudden Na24 volume increase and a decrease of plasma volume. The rates of change of all indices were greater in the thyroidectomized group. Thyroidectomized animals were characterized by a more rapid rate of cooling, most marked in peripheral areas, and Na24 dilution volumes indicated a greater extravascular volume of fluid entered by the sodium isotope. It is concluded that thyroidectomy prior to cold exposure makes heat conservation responses less efficient than in the normal animal and that the intact thyroid plays a role in maintaining body fluids in an efficient equilibrium for rapid adjustment to a cold environment.