Thyroid hormones in chronic heat exposed men

Previous reports have indicated that thyroid gland activity, is depressed in the heat. Total thyroxine (T₄) and triiodothyronine (T₃) serum levels in 17 workers of the metal work shop at a plant near the Dead Sea and 8 workers in Beer Sheva, Israel were examined. The metal workshop of the plant near the Dead Sea is part of a large chemical plant. The one in Beer Sheva is part of a large construction company. Maintenance work, as well as metal work projects are performed in both workshops. During the work shifts, the workers of the Dead Sea plant were exposed to temperatures ranging from 30-36°C (May-Oct.) and 14-21°C (Dec.-Feb). In Beer Sheva the range was 25-32°C (June-Sept.) and 10- 17°C (Dec.-Feb.). Total T₄ was measured by competitive protein binding and total T₃ by radioimmunoassay in blood drawn before work (0700) in July and January. In summer. T₄ was higher and T₃ was lower for both groups than in winter. The observed summer T₃ decrease may result from depressed extrathyroidal conversion of T₄ to T₃. We conclude that the regulation of energy metabolism in hot climates may be related to extrathyroidal conversion of T₄ to T₃.