Amyloid fibrils are found in almost all aspects of life and are often associated with neurotoxic effects as in e.g. Alzheimer's disease. Exemplary, fibrils can also perform functional tasks, including the storage of peptide hormones in secretory granules. In this dissertation, amyloid fibril formation of the parathyroid hormone PTH84 was investigated with respect to its molecular mechanism. A complex relationship between fibrillation process and a peptide concentration dependent prenucleation equilibrium was identified. Furthermore, important insights into the critical concentration as a threshold for certain oligomers, which can induce the formation of amyloid fibrils by a structural conversion, could be obtained. These may contribute to a better understanding of metastable supersaturated peptide and protein solutions. In addition, this work provides a contribution to the classification of PTH84 as a functional amyloid.