Introduction: The pathology of thyroid gland occupies the chief place in the structure of the diseases of the endocrine system. The success of the therapeutic tactics of these kind of patients depends not only on precise diagnostics and types of focal changes in thyroid tissue, but on relevant estimation of its functional state. There are numerous methods of estimation of focal and morphological changes in thyroid gland (ultrasound, FNAB, scintigraphy) described in a literature. In spite of this, the evaluation of thyroid functional activity still remains complicated, expensive and, sometimes, long-lasting process. That is why the search of new, more simple and cheap methods of estimation of thyroid functional activity is feasible and fulfills the present necessity. Materials & Methods: We have proposed the spectrophotometric method, due to which one can estimate the optical density of blood plasma. This parameter shows the concentration of different optical structures of blood plasma due to various wavelengths. It was found that the most probable changes of plasma optical density in case of various surgical diseases are detected on 280nm wavelength. This wavelength surely may characterize the protein fractions of blood globulins, which take part in regulation of thyroid homeostasis. We have studied the possible administration of the blood optical density values for the diagnostics of thyroid severity grades in patients with diffent forms of thyrotoxic goiter before the surgery. The total amounts of 47 patients with different forms of thyrotoxic goiter were examined. The patients were distributed into 3 groups: 1 group (N-15) with thyrotoxicosis of low grade, 2 group (N-18) of moderate and 3 group (N-14) of sever grade respectively. The diagnosis was set due to clinical protocols and laboratory tests for evaluation of thyroid homeostasis. The control group was of 20 practically healthy people. Results: During the examination of optical blood plasma density on the 280 nm wavelength it was detected the value of 0,58±0,01 units in control group. In patients with low grade thyrotoxicosis this value was 0,71±0,02 units and 30-32% higher comparing the control. In patients with moderate grade thyrotoxicosis this value was 0,75±0,01 units and 38-42% higher comparing the control. In patients with severe grade thyrotoxicosis this value was 0,79±0,01 units and 45% higher comparing the control respectively. Conclusions: Highly relevant differences in blood plasma optical density in patient with thyrotoxic forms of goiter may serve as additional diagnostic criteria of thyrotoxic syndrome. Moreover, the clinical administration of this method evaluates the treatment’s efficacy.