Causes of difficulties with adequate levothyroxine substitution — an immunoendocrine perspective

Hypothyroidism is one of the most common endocrinopathies worldwide, the treatment of which is based on replacement therapy with levothyroxine. However, this seemingly simple treatment method is fraught with many difficulties and frequent dissatisfaction among patients. In fact, differences in response to levothyroxine probably depend on a complex interaction between individual, environmental, genetic, and epigenetic factors that are still not sufficiently understood. Immunological disturbances, underlying Hashimoto's disease, the most common cause of hypothyroidism, probably play a significant role in these relationships. Indeed, a growing number of studies indicate that autoimmunity through activation of low-grade inflammation can lead to impaired absorption, transport, metabolism, and action of thyroid hormones. This review provides an up-to-date overview of the causes responsible for both the difficulty in achieving target thyrotropin levels and persistence of nonspecific symptoms despite adequate hormone replacement from an immunoendocrine perspective. Understanding these mechanisms points to a new direction in the approach to hypothyroidism, indicating the need for new personalized treatment strategies.