NOMOTHETICOS: Nonlinear Modelling of Thyroid Hormones' Effect on Thyrotropin Incretion in Confirmed Open-loop Situation

Inclusion Criteria: * Outpatients with disconnected feedback control due to the following conditions: * Overt primary hypothyroidism with TSH level higher than 10 mU/l and FT4 concentration below 7 pmol/L (5.4 ng/L) (Partition 1) * Overt primary hyperthyroidism with TSH level below 0.1 mU/l and FT4 concentration higher than 18 pmol/L (14 ng/L) (Partition 3) * All other constellations, if the patient receives substitution therapy with more 1.75 µg Levothyroxin per kg of body mass (Partition 2). * System in equilibrium (e.g. unchanged substitution dose over the past six weeks) Exclusion Criteria: * Pituitary or hypothalamic dysfunction * Severe illness that may be associated with euthyroid sick syndrome (non-thyroidal illness syndrome) * Medication influencing pituitary function * Pregnancy * Missing consent for participation in the study