[Substitution of thyroid hormones].

In patients with overt hypothyroidism the substitution with Levothyroxine is mandatory, whereas in patients with subclinical hypothyroidism it is dependent on further clinical signs and symptoms. The main parameter indicating sufficient substitution of Levothyroxine in patients with primary hypothyroidism is TSH, whereas in secondary hypothyroidism it is free T(4). At present Levothyroxine is the preparation of choice to restore euthyroidism in hypothyroid patients because it is assumed that the peripheral conversion from T(4) to T(3) provides the exact amount of T(3) needed by every organ. Intestinal absorption of Levothyroxin is around 80%, but only in acid environment and is significantly decreased by proton-pump inhibitors, antacids, atrophic gastritis and Helicobacter pylori infection. The usual required dosage of Levothyroxine is 1.5 microg/kg of body weight but is increased under these conditions as well as during early childhood and pregnancy, and is decreased in older people. The bioavailability of different Levothyroxin generics might change between 0.8 and 1.25. Therefore, the thyroid function of patients, receiving another brand of Levothyroxine has to be re-checked.