Environmental influences on hypothalamic-pituitary-thyroid function and behavior in Antarctica.

We examined the physiological and psychological status of men and women who spent the summer (n=100) and/or winter (n=85) seasons in Antarctica at McMurdo (latitude 78.48 S, elevation 12 m) and South Pole (latitude 90 S, elevation 3880 m) stations to determine whether there were any significant differences by severity of the stations' physical environment. Physiological measures (body mass index, blood pressure, heart rate, tympanic temperature), serum measures of thyroid hormones, cortisol, and lipids and plasma catecholamines were obtained at predeployment (Sep-Oct) and the beginning of the summer (November) and winter (Mar-Apr) seasons. Cognitive performance and mood were assessed using the Automatic Neuropsychological Assessment Metric - Isolated and Confined Environments (ANAM-ICE), a computerized test battery. South Pole residents had a lower body mass index (p<0.05) and body temperature (p<0.01) and higher levels of plasma norepinephrine (p<0.05) in summer and winter than McMurdo residents. Upon deployment from the United States and during the summer, South Pole residents experienced significantly higher thyroid hormone values (free and total T(3) and T(4)) (p<0.01) than McMurdo residents; in summer they also experienced lower levels of triglycerides (p<0.01) cortisol (p<0.05) and LDL (p<0.05). In winter, South Pole residents also experienced a 39% decrease in serum TSH compared with a 31.9% increase in McMurdo (p<0.05). South Pole residents also were significantly more accurate (p<0.05) and efficient (p<0.01) in performance of complex cognitive tasks in summer and winter. Higher thyroid hormone levels, combined with lower BMI and body temperature, may reflect increased metabolic and physiological responses to colder temperatures and/or higher altitude at South Pole with no apparent adverse effect on mood and cognition.

Psychoneuroendocrine effects of combined thyroxine and triiodothyronine versus tyrosine during prolonged Antarctic residence.

OBJECTIVES: We previously reported that cognitive function improves with thyroxine and that there is a circannual pattern to mood and human TSH during Antarctic residence. To extend these findings, we examined the effects of tyrosine and a combined levothyroxine/liothyronine supplement in euthyroid men and women who spent the austral summer (n = 43) and/or winter (n = 42) in Antarctica. STUDY DESIGN: Randomized, placebo-controlled, clinical trial. METHODS: Subjects were randomized to receive the following each day for 91.6 +/- 3.2 days in summer and/or 138.0 +/- 3.2 days in winter: (1) 12g tyrosine mixed in 113g applesauce; (2) 50 microg of levothyroxine and 12.5 microg of liothyronine (T4-T3 Supplement); or (3) placebo. Cognitive performance and mood were assessed using the Automatic Neuropsychological Assessment Metric - Isolated and Confined Environments. RESULTS: With placebo in summer, mood did not change while TSH decreased by 28%; in winter, there was a 136% degradation in mood (p < 0.01) and TSH increased by 18%. With combined T4-T3 supplement, there was a 51% degradation in mood in summer compared with placebo (p < 0.05) and TSH decreased by 57%; in winter there was a 135% degradation in mood while TSH was reduced by 26% (p < 0.05). Tyrosine use in summer was associated with no change in mood and a 30% decline in TSH, while in winter there was a 47% improvement in mood and TSH decreased by 28% along with a 6% increase in fT3 (p < 0.05). CONCLUSIONS: Administration of tyrosine leads to a significant reduction in serum TSH and improvement in mood in winter compared with placebo, while the combined T4-T3 supplement leads to a worsening of mood in summer and no improvement in winter. There appears to be a seasonal influence on the psychological response to interventions and the relationship to changes in TSH reductions.

Resting and exercise energy use in Antarctica: effect of 50% restriction in temperate climate energy requirements.

OBJECTIVE: To determine the impact of energy restriction (ER) upon the previously reported increased resting and exercise-related oxygen utilization, reduced body temperature, increased serum TSH, and reduced serum free T3 concentrations found during Antarctic residence (AR). DESIGN: Prospective, intervention with both paired controls and a similar reference control group (RG). PATIENTS AND MEASUREMENTS: Seven subjects were assessed before and after a 50% ER period of 60 h. This ER was carried out within 30 days of arriving in Antarctica in October (OCT) and again after 10 months AR in August (AUG). During the periods of ER, mean energy consumption was 5662 +/- 1344 kJ/day in OCT and 5529 +/- 967 kJ/day in AUG. Resting metabolic rate (RMR), a calculated resting metabolic rate (RMR(reg)) using a submaximal work regression, serum TSH, FT3 and tympanic temperature (Tty) were measured. These values were compared with a similar RG of 12 subjects reported previously who were studied in California, USA before and then again during AR. RESULTS: Weight declined by 1.1 +/- 0.1 kg/day (OCT) and 0.92 +/- 0.2 kg/day (AUG) with ER, resulting in a reduction of body weight by 3.1 +/- 0.4% in OCT (P = 0.0001) and 2.5 +/- 0.4% in AUG (P = 0.0015) during AR. The RMR before ER did not change with AR and it was not significantly different from the RG studied in California. With ER the RMR tended to decline in both OCT (132 +/- 5 to 122 +/- 4 mlO2/min/m2) and AUG (134 +/- 5 to 126 +/- 5 mlO2/min/m2), but these were not significant. By contrast, RMR(reg) obtained before ER was increased with AR by 22.5 +/- 7.8% (P = 0.01) in OCT and by 28.1 +/- 7.0% (P = 0.0008) in AUG over the RG values obtained in California. RMR(reg) did not decrease with ER in either OCT or AUG. The total energy expenditure derived from these measures of weight loss suggests that 24-h energy requirements are 74.4%[95% confidence interval (CI) 2.6-146.3; P < 0.05] more than those expected in temperate climates. Tty declined by 0.6 +/- 0.2 degrees C (P < 0.01) with AR compared with the RG measured in California, but was not affected by either period of ER. ER had no effect on FT3 but tended to decrease serum TSH in AUG (P = 0.06). CONCLUSIONS: Exercise-related energy requirements are increased with AR. Moderate ER may reduce resting but not exercise-related energy expenditure and it is associated with a weight loss exceeding expectations for 50% restriction of temperate climate energy predictions.

Elevation in serum thyroglobulin during prolonged Antarctic residence: effect of thyroxine supplement in the polar 3,5,3'-triiodothyronine syndrome.

Extended Antarctic residence (AR) is associated with an increase in serum TSH, a decrease in free T(4), and an increase in T(3) production and clearance. It is not clear whether these adaptations reflect changes in clearance alone or whether intrinsic thyroidal synthetic activity also changes. Thyroglobulin (Tg) secretion is an independent marker of intrinsic thyroid activity whose kinetics are independent of those of T(3) and T(4). In this study we examined changes in Tg levels in healthy subjects before and during AR and their responses to thyroid supplementation to help determine whether alterations in thyroid activity, and not just kinetics of clearance, underlie the changes seen with the polar T(3) syndrome. In cohort 1, we compared measurements of TSH and Tg in 12 subjects before deployment and monthly for 11 months during AR. In cohort 2, we compared the same measurements in 12 subjects monthly for 11 months of AR. Subjects were randomized to receive either placebo or levothyroxine in cohort 1 for 7 months and in cohort 2 for 11 months. Tg increased over baseline during the first 4 months of AR by 17.0 +/- 4.6% and after 7 more months by 31.7 +/- 4.3% over baseline in the placebo group of both cohorts (P < 0.0002). When L-T(4) was taken, Tg returned to a value not different from baseline (4.5 +/- 3.9%). The percent changes from baseline in serum TSH and Tg during AR were highly correlated (P < 0.00003) in the placebo group for both cohorts. The rise in Tg with TSH and the reduction in Tg with L-T(4) provide evidence of target tissue response to TSH and further confirm the TSH rise as physiologically significant. The results also suggest that the adaptive changes in thyroid hormone economy with AR reflect TSH-dependent changes in thyroid synthetic activity, which may help explain a portion of the increases in T(3) production found with AR.

Change in serum sodium concentration during a cold weather ultradistance race.

OBJECTIVE: To investigate change in serum sodium concentration and its potential causes during a cold weather ultradistance race. DESIGN: Descriptive research. SETTING: A 100-mile (161-km) race over a snow-packed course in the Alaskan wilderness. Athletes competed in 1 of 3 divisions: foot, bike, or ski. PARTICIPANTS: Twenty athletes (11 runners, 6 cyclists, 3 skiers) volunteered for the study. INTERVENTIONS: None. MAIN OUTCOME MEASURES: Subjects were weighed and had blood drawn for hematocrit, serum sodium, serum aldosterone, and plasma arginine vasopressin concentrations pre- and postrace. Fluid and sodium intake were determined by race dietary analysis. RESULTS: Serum sodium concentration decreased significantly prerace (140.8 +/- 1.2 mmol/L) to postrace (138.4 +/- 2.2 mmol/L), although no athletes were classified as hyponatremic. Mean weight loss was significant during the race (-1.2 kg), although 1 athlete maintained his weight, and 3 athletes gained small amounts of weight (0.2 kg, 0.2 kg, and 0.5 kg, respectively). Hematocrit decreased significantly prerace (42.2 +/- 3.5) to postrace (40.3 +/- 4.1). Plasma arginine vasopressin and serum aldosterone increased significantly during the race (2.6 +/- 0.7 to 6.0 +/- 4.6 pg/mL and 5.1 +/- 2.6 to 40.8 +/- 25.1 ng/dL, respectively). Fluid consumption was 300 +/- 100 mL/h, and sodium intake was 310 +/- 187 mg/h. CONCLUSIONS: Decreased serum sodium concentration after a cold weather ultradistance race was due to fluid overload caused by excessive fluid consumption. Current recommendations that ultradistance athletes consume 500 to 1000 mL/h may be too high for athletes competing in the extreme cold.

Hyponatremia in a cold weather ultraendurance race.

We assessed the incidence and etiology of hyponatremia in the 100-mile (161 km) Iditasport ultramarathon. Subjects (8 cyclists, 8 runners) were weighed and serum sodium was measured pre- and post-race. Race diets were analyzed to determine fluid and sodium consumption. Subjects were split by post-race serum sodium concentration into hyponatremic and normonatremic groups for statistical analyses. Seven of 16 subjects (44%) were hyponatremic. The hyponatremic group exhibited a significant decrease in serum sodium concentration (137.0 to 132.9 mmol/L), and the normonatremic group experienced a significant decrease in weight (82.1 to 80.2 kg) pre- to post-race. The hyponatremic group drank more fluid per hour (0.5 versus 0.4 L/h) and consumed less sodium per hour (235 versus 298 mg/h) compared to the normonatremic group. In conclusion, hyponatremia is common in an ultraendurance race held in the extreme cold, and may be caused by excessive fluid consumption and/or inadequate sodium intake.