The thyroid hormones, thyroxin (T4) and triiodothyronine (T3), are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxin (T4), which has a longer half-life than T3. The ratio of T4 to T3 released into the blood is roughly (20 to 1). The level of thyroxin is affected by radiation. The aim of the study to determine and to assess the effects of continuous X-rays on thyroxin level in vitro quantitive measures. Thirty patients with hyperthyroidism disease were enrolled in this study. Blood samples were tested and irradiated by x-ray radiation source of total dose rate (0.4).Gry/sec. The level of thyroxin were determine before and after irradiation .The results showed that the level.of.thyroxine.are.significantly.reduced.(P<0.01).after.x-ray.irradiation. It is concluded that x-ray ionizing radiation, reduced thyroxin level by free radical forming and protein damaging.