This is a case report of a female patient who developed, after radioactive iodine therapy for Grave's hyperthyroidism, a severe clinically manifested hypothyroidism, which was accompanied by predominantly sensory neuropathy of dominant axonal type. Besides symptoms and neurophysiological signs of polyneuropathy, the patient, in spite of seemingly adequate levothyroxine monotherapy for hypothyroidism during the last 2 years, still manifested persistent hypothyroid symptoms by CNS, higher serum concentrations of total and LDL cholesterol and clinical signs of the increased peripheral vascular resistance. Non-physiological serum FT3 and FT4 ratio was maintained, together with serum TSH and FT4 concentrations, which suggested lower sensitivity of thyroid-hypophyseal negative feedback. The introduced combined LT4/LT3 substitution therapy resulted in quick subsidence of CNS-related hypothyroid symptoms, significant reduction of serum concentrations of total and LDL cholesterol, FT3/FT4 ratio normalization, at least in the first hours of drug administration, and disappearance of clinical signs of the increased peripheral vascular resistance. The extent of polyneuropathic difficulties was reduced to the level of not disturbing the sleep. Time and control ENG will show the final effects of combined T4/FT3 therapy on the peripheral nerve injuries. In the first 6 months, no adverse effects of this therapy were recorded.