The sensitive equilibrium of the thyroid hormone loop can be disturbed due to the presence of different autoantibodies. Autoimmune thyroid diseases are characterised by antibodies against thyroid microsomes, whose main target antigen is thyroperoxidase (TPO), and antibodies against thyroglobulin (TG) or the receptor of thyrotropin (also known as thyroid stimulating hormone (TSH). The most frequent autoimmune thyroid diseases are Hashimoto’s autoimmune thyroiditis (AIT) and immunohyperthyroidism, also known as Graves’ disease. While AIT can manifest as a thyroid overfunction (hyperthyroidism) or underfunction (hypothyroidism), Graves’ disease is always associated with hyperthyroidism.
Hashimoto’s thyroiditis leads to autoimmune-mediated lymphocytic infiltration and consequently to a destruction of the thyroid tissue, in the long term often resulting in reduced thyroid hormone production.
A special form of autoimmune thyroiditis is post-partum thyroiditis, a temporary hypothyroid functional disorder of the thyroid accompanied by high titers of anti-TPO antibodies. The disease affects around 5 to 9% of women after giving birth, and the risk is especially high in diabetes mellitus patients.
The permanent stimulation of TSH receptors by binding of autoantibodies is one of the most relevant factors for the pathogenesis of Graves’ disease. The permanent binding activates signal cascades leading to an increased uptake of iodine by the thyroid. As a consequence, the thyroid hormones triiodothyronine (T3) and thyroxine (T4) are produced and secreted in larger quantities.
In suspected cases of thyroid disease, the TSH concentration in serum should be determined. An increased TSH level indicates hypothyroidism, low values indicate hyperthyroidism. Additionally, the values of the free thyroid hormones FT3 or FT4 in serum should be determined. The detection of antibodies against thyroid antigens enables differentiation of autoimmune thyroid diseases from acute (bacterial) or subacute (non-infectious) thyroiditis or non-autoimmune disturbed thyroid hormone regulation.
With Hashimoto’s thyroiditis, anti-TPO antibodies are detected in 90% of patients; autoantibodies against TSH receptor (TRAb) are found in 6 to 60%, and anti-TG antibodies occur in 45 to 60% of cases.
TRAb are the most important serological marker in Graves’ disease. They are detectable in over 90% of patients. Higher antibody titers are associated with severe disease courses. Moreover, anti-TPO and anti-TG antibodies occur with a prevalence of approx. 80% and 30%, respectively.
For sufficient differential diagnostics, the overall picture obtained from the investigation of different parameters must be evaluated. Besides serological analyses, clinical examinations (e.g. ultrasound, scintigraphy) and symptoms must be taken into account.