Thyroid activity is regulated by the thyroid-stimulating hormone (TSH) secreted by pituitary. Elevated TSH levels induce the thyroid to elaborate thyroxine, a hormone which functions in at least 20 enzyme systems; one of its major influences involves the acceleration of protein synthesis. High-frequency EMFs seem to have a general stimulatory effect on the thyroid. At 70 MHz, I50 v/m, 3 months' exposure resulted in an increase in the height of the follicular epithelium in rats-there was no change in thyroid weight (14) . At 3 GHz, I 53 ,µW/cm2, an increase in thyroid weight was found after 2 weeks' exposure, but after 5 months' exposure the thyroid weights were normal (12). Following 4 months' exposure to 5000 W/crn~, cellular incorporation of radioactive iodine and serum proteinbound iodine were increased by 50 and II7%, respectively (16). Electron micrographs revealed enhanced cellular activity that was manifested by an increased number of cytosomes and an enlarged Golgi apparatus and endoplasmic reticulum (16).
At 50 Hz thyroid activity was depressed as judged by radioactive iodine uptake (7, 17). Continuous exposure at 1-5 kv/m depressed thyroid activity after 4 months (7): when the field was removed thyroid activity returned to normal within 6 weeks. Four months intermittent exposure (2 hr/day) at the same field level did not affect thyroid activity, but depressed activity was observed at 7-15 kv/m (17). Ossenkopp et al. found that both male and female rats exposed in utero to 0.5 Hz, 0.5-30 gauss, had increased thyroid weights at 105-130 days of age (21). Based on this and several other physiological and behavioral studies, Persinger has implicated the thyroid as a significant factor in the rat's response to a magnetic field (22).