Abstract
BACKGROUND: It is known that subclinical hypothyroidism (SCH) often converts to euthyroidism. However, the mechanism of such changes is unclear and can only be speculated. This difficulty is likely due to limitations in diagnostic guidelines and their interpretation. Therefore, in the presented case, important diagnostic criteria that can improve understanding of the essence and pathogenesis of primary hypothyroidism are shown and explained. CASE DESCRIPTION: A 15-year-old female patient experienced a transition from SCH to euthyroidism against the background of highly normal values of thyroid hormones, the full state of the thyroid parenchyma and significantly increased blood flow in the Doppler mode. She did not take medications. Conditions likely to trigger SCH included acute respiratory illness, living in a cold climate, and mental stress due to prolonged preparation for exams. The transition to euthyroidism occurred after sufficient stay in the conditions of a southern resort in the summer and mental comfort. CONCLUSIONS: As a result, it was revealed that the amount of thyroid-stimulating hormone (TSH) depends on the intensity of energy expenditure of the body in accordance with living conditions. The increased intensity of blood flow in the thyroid during SCH and euthyroidism shows the leading role of the autonomic nervous system and the auxiliary importance of TSH. The concentration of thyroid hormones at the maximum limit of normal in SCH, together with the significantly increased Doppler blood flow, suggests common elements of pathogenesis with Graves' disease through the autonomic nervous system. With many months of thyroid overstrain, ultrasound shows signs of corresponding depletion in the form of hypoechogenicity of the lobules, but the preservation of a sufficient amount of hormone-producing tissue.