Reduced thermoregulatory null zone in postmenopausal women with hot flashes

OBJECTIVE: Most menopausal hot flashes are preceded by small elevations in core body temperature. If the thermoneutral zone between the thresholds for sweating and shivering is reduced in women with symptoms, the triggering mechanism for hot flashes could be explained. STUDY DESIGN: We studied 12 postmenopausal women with symptoms and 8 without symptoms. We measured body temperatures with a rectal probe, an ingested telemetry pill, and a weighted average of rectal and skin temperatures. Each woman underwent 3 experimental sessions: determination of the sweating threshold by body heating, determination of the shivering threshold by body cooling, and replication of the sweating threshold with exercise. RESULTS: The women with symptoms had significantly smaller interthreshold zones than did the symptom-free women for all 3 measures of body temperature: rectal temperature, 0.0 degrees C +/- 0.06 degrees C versus 0.4 degrees C +/- 0.18 degrees C (P < .005); telemetry pill temperature, 0.0 degrees C +/- 0.11 degrees C versus 0.4 degrees C +/- 0.18 degrees C (P < .005); and mean body temperature, 0.8 degrees C +/- 0.09 degrees C versus 1.5 degrees C +/- 0.20 degrees C (P < .0006). Sweat rates were significantly higher among the women with symptoms (0.06 +/- 0.002 mg.cm(-2).min(-1)) than among the women without symptoms (0.03 +/- 0.001 mg.cm(-2).min(-1), P < .05). Sweating thresholds during exercise did not significantly differ from those during body heating. During exercise all the women with symptoms and none of the women without symptoms had hot flashes. CONCLUSIONS: Menopausal hot flashes in women with symptoms may be triggered by small elevations in body temperature acting within a reduced thermoneutral zone.