Formaldehyde (HCHO) exposure in industry causes health impairment, especially immune profile. Male albino rats when exposed to HCHO at 5, 10, 15 mg/kg day for 30 days showed decrease in thyroid weight in 10, 15 mg/kg/day HCHO fed group with commensurate follicular regression (epithelial cells), decreased triiodothyronine (T3), thyroxin (T4) deficiency and enhanced thyroid stimulating hormone (TSH). Repeated chemical stimulation might have increased thyroid activity of the follicles and rapidly deteriorated the capacity for synthesis of thyroxin hormone and thus becomes atrophied. The chronic action of HCHO on thyroid may be due to a block at the source of thyroid hormone (s).