Transthyretin (TTR) is an amyloidogenic protein whose aggregation is responsible for several familial amyloid diseases. Here, we use FTIR to describe the secondary structural changes that take place when wt TTR undergoes heat- or high-pressure-induced denaturation, as well as fibril formation. Upon thermal denaturation, TTR loses part of its intramolecular β-sheet structure followed by an increase in nonnative, probably antiparallel β-sheet contacts (bands at 1616 and 1686 cm −1) and in the light scattering, suggesting its aggregation. Pressure-induced denaturation studies show that even at very elevated pressures (12 kbar), TTR loses only part of its β-sheet structure, suggesting that pressure leads to a partially unfolded species. On comparing the FTIR spectrum of the TTR amyloid fibril produced at atmospheric pressure upon acidification (pH 4.4) with the one presented by the native tetramer, we find that the content of β-sheets does not change much upon fibrillization; however, the alignment of β-sheets is altered, resulting in the formation of distinct β-sheet contacts (band at 1625 cm −1). The random-coil content also decreases in going from tetramers to fibrils. This means that, although part of the tertiary- and secondary-structure content of the TTR monomers has to be lost before fibril formation, as previously suggested, there must be a subsequent reorganization of part of the random-coil structure into a well-organized structure compatible with the amyloid fibril, as well as a readjustment of the alignment of the β-sheets. Interestingly, the infrared spectrum of the protein recovered from a cycle of compression-decompression at pD 5, 37°C, is quite similar to that of fibrils produced at atmospheric pressure (pH 4.4), which suggests that high hydrostatic pressure converts the tetramers of TTR into an amyloidogenic conformation.