Background We aimed to estimate lifetime cumulative risk of thyroid cancer (CRTC) in first-degree relatives of patients with non-medullary thyroid cancers (NMTC), including papillary (PTC)/follicular/oxyphilic/anaplastic thyroid carcinoma, by histology and age at diagnosis in patients and their relatives.

Design A population-based cohort of 63 495 first-degree relatives of 11 206 NMTC patients diagnosed in 1955–2009 in Nordic countries was followed for cancer incidence. Standardised incidence ratios (SIRs) were calculated using histology-specific, age-specific, sex-specific, period-specific and country-specific incidence rates as reference.

Results The 0–84-year CRTC in female relatives of a patient with PTC was 2%, representing a threefold increase over the general population risk (SIR=2.9, 95% CI 2.4 to 3.4; Men: CRTC=1%, SIR=2.5, 95% CI 1.9 to 3.3). When there were ≥2 PTC patients diagnosed at age <60 years in a family, CRTC for female relatives was 10% (male 24%). Twins had a 23-fold increased risk of concordant PTC. Family history of follicular/oxyphilic/anaplastic carcinoma increased CRTC in relatives to about 1–2%. Although no familial case of concordant oxyphilic/anaplastic carcinoma was found, familial risks of discordant histology types of NMTC were interchangeably high for most of the types, for example, higher risk of PTC when a first-degree relative had follicular (SIR=3.0, 95%CI 1.7 to 4.9) or anaplastic (SIR=3.6, 95% CI 1.2 to 8.4) carcinoma. The earlier a patient was diagnosed with PTC in a family, the higher was the SIR in his/her younger relatives. There was a tendency towards concordant age at diagnosis of thyroid cancer among relatives of PTC patients.

Conclusions This study provides clinically relevant risk estimates for family members of NMTC patients.