Observational Study of Neurofilament Light Chain (NfL) as a Biomarker in Asymptomatic Carriers of the Transthyretin (TTR) Variants and Patients With Hereditary Transthyretin-mediated (hATTR) Amyloidosis With Polyneuropathy

Inclusion Criteria: \- Carrier of a documented pathogenic TTR variant confirmed with genotyping with predicted disease onset within 5 years and not diagnosed with hATTR amyloidosis with polyneuropathy OR Confirmed diagnosis of hATTR amyloidosis with polyneuropathy with a documented TTR variant confirmed with genotyping \- Participant is able to understand the study and does not oppose participating in the study after reviewing the content of the PIS provided. Exclusion Criteria: * A known condition (other than hATTR amyloidosis) that can cause nerve damage and affect NfL levels * Estimated glomerular filtration rate (eGFR) \<45 milliliters per minute per 1.73 meters squared (mL/min/1.73 m\^2) * Currently enrolled in a clinical study for any investigational agent.