In order to evaluate the pineal-thyroid-pituitary interaction, the serum melatonin concentration was estimat­ed in 45 age and sex matched sub­jects: 20 with hyperthyroidism, 12 with hypothyroidism and 13 with euthyroid function. Serum melatonin as well as serum ts, 14, and TSH were deter­mined by enzyme immunoassay. A significant decrease in serum melato­nin was observed in hyperthyroid pa­tients whereas there was a significant increase of serum melatonin in hypo-thyroid patients. Also, there was a sig­nificant negative correlation between serum melatonin and serum T4 in all subjects. On the other hand, there was a significant positive correlation between serum melatonin and serum TSH in euthyroid idividuals. In conclu­sion our study demonstrated that pa­tients with hyperthyroidism secrete less melatonin than euthyroid sub­jects and this observation may help in ameliorating the anxiety symptoms that accompany thyrotoxicosis.