Thyroid morphology and wet organ weight changes in rats exposed to different low intensity 0.5 Hz magnetic fields and pre-experimental caging conditions

In eight separate experiments, 48 adult female rats were caged in groups of 3 or 4 individuals under normal lighting conditions for 120–140 days. They were then exposed in pairs to one of 6 different variation intensities (0.001 to 10 G, i.e., 10^–7 T to 10^–3 T) of a 0.5 Hz rotating magnetic field (RMF) or to one of 3 sham field positions for 10 days while under constant 45 lux illumination. Post-mortem examination and histological analyses demonstrated that RMF-exposed rats show no significant changes in thyroid follicle numbers, perifollicular mast cell numbers, wet adrenal and pituitary weights, body weight changes, or water consumption either between intensity groups or relative to sham field controls. Significant interactions existed for thyroid and spleen weights between exposure intensities and preexposure caging conditions for the 10^–5 T to 10^–6 T groups; the effect was not linearly related with field intensity. Rats that had been maintained in the 4/cage situation showed significantly lighter body weights and more thyroid follicles; the significant alterations in thyroid, adrenal and spleen weights were related to body weight.