Figure 1. The figure describes the transition from hypothyroidism to hyperthyroidism during follow up in the same patient. The phenomenon is due to a shift in the concentrations and affinities of TBAb and TSAb. In our series the mean time for this switch was of 38 months. Thyrotropin (TSH) has a constitutive activity, as shown by the horizontal line through the wedge. TSHR ligand (TSH or TSAb) further increase receptor activity. An inverse agonist supresses constitutive activity, whereas a full agonist maximally activates the receptor. Ligands of intermediate activity are either neutral agonists or partial agonists or inverse agonists. Ligands can also be an antagonist depending on their relative affinities and binding sites. Therefore, TSAb beeing a partial agonist may also be an antagonist for TSH. If a serum displays both TSAb and TBAb activity, unless the former is very weak and the latter is very strong, it cannot be assumed that there are two separate antibodies. [Adapted from: McLachlan et al. (13)].