Abstract
Measurements of fluid and caloric intake, urine output, urine osmolality, creatinine clearance, weight change, 48-hour PBI and thyroid radioiodine clearance, 4-hour hematocrit increment, and 12-hour urinary 17-OH-CS excretion have been conducted in newborn term infants exposed to relatively cold ambient temperature (23-29°C) and contrasted with values in infants incubated at ambient air temperatures of 31-40°C during the first 36-48 hours of life. Deep rectal and skin temperatures were consistently lower in the infants in the cooler environment. Non-incubated infants showed significantly higher PB! values and higher thyroid radioiodine clearance at 48 hours than did the incubated infants, suggesting that the usual feedback suppression of TSH secretion is inoperative in the infants in the colder environment. These observations are in agreement with the hypothesis that the neonatal hyperthyroid state is the result of thermal receptor stimulation of TSH secretion by the relatively cold extrauterine environment. The cooler non-incubated infants showed significantly greater urine volumes with significantly lower (urine) osmolality than did the incubated infants. Mean osmolal and creatinine clearance were similar in the two groups of infants. Although this cold induced water diuresis seems most likely due to a larger fluid intake in the non-incubated infants in response to a greater metabolic rate, the observation of similar non-renal water metabolism in cold and warm infants suggests the possibility of a primary "cold diuresis" similar to that observed during acute cold exposure in adults. A significantly greater increment in hematocrit was observed during the first 4 hours in the colder infants. The mechanism of this response, presumably due to a displacement of plasma volume from the vascular compartment, remains to be defined. Urinary methylene chloride extractable 17-OH-CS excretion was similar in the cold exposed and the incubated infants.
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