Introduction: Exposure to cyanide is encountered in many industries as in Case Hardening and Electroplating. Chronic exposure to cyanide is associated with some health effects including its effect on thyroid gland. Aim of work: To study the health effects due to chronic occupational exposure to cyanide with special emphasis on the thyroid gland. Materials and Methods: This study was conducted on 56 workers exposed to cyanide in case hardening and electroplating industry. Their age ranged between 36 and 59 years with a mean of 46.96 ± 5.51 years. They are exposed to cyanide from 4 to 31 years with a mean of 19.86 ± 8.53 years. They were compared with 60 workers not exposed to cyanide as a control group, Their age ranged between (30-60) years with a mean of 45.76 and S.D. ±11.81. Hydrogen cyanide was measured in air of workplace environment. Serum thiocyanate serum and thyroid hormones (T3, T4, and TSH) were measured. Results: The serum thiocyanate levels showed highly statistically significant difference among exposed workers when compared to the control. A highly statistically significant lower serum levels of T3 and T4 among the exposed group, and higher serum levels of TSH compared to the control group. Serum TSH showed a statistically significant positive correlation with serum thiocyante among the exposed workers, a statistically significant negative correlation as regards T3, and a negative correlation between TSH and T4 but it is not statistically significant. Conclusion: Cyanide exposure is associated with decrease in thyroid functions but not clinically detected.