Editor—Incidence of thyroid carcinoma in Iceland is high1 and papillary carcinoma comprises 80% of all thyroid malignancies in the country.2 It is well known that medullary thyroid carcinoma aggregates in families and the genetic component in the aetiology has been established.3 Familial occurrence of non-medullary thyroid carcinoma is rare, but it has been increasingly recognised in recent years.4-6 There has been increasing awareness that this may constitute a distinct disease subgroup rather than chance occurrence, as reflected by the review of case series by Loh.7 Familial papillary microcarcinoma has also been suggested as a new clinical entity.8 9 In an earlier paper, we reported on the occurrence of papillary thyroid carcinoma (373 probands) in first degree relatives of all patients diagnosed between 1955 and 1984 in Iceland.10 This study showed a non-significant trend to familiality. In the present study, the population includes second and third degree relatives in addition to first degree relatives and we now include all non-medullary thyroid carcinoma patients.

Information on thyroid cancer cases was obtained from the Icelandic Cancer Registry. All cases (n=712) of non-medullary thyroid carcinoma diagnosed in Iceland between 1955 and 1994 form the basis of this study. This includes 147 thyroid cancers found incidentally at necropsy and 565 thyroid cancers diagnosed clinically. The information on first, second, and third degree relatives was obtained from the Genetical Committee of the University of Iceland. This committee has a computerised family tracing resource based on the 1910 census and birth records for 1840-1910. This resource is linked with the national register and death records for later periods. Permission was obtained from the Data Protection Commission to link files from …