Familial relationships in thyroid cancer by histo-pathological type.

Thyroid cancer was studied in the Swedish Family-Cancer Database, which was updated in 1999 to cover individuals born after 1934 with their biological parents, for a total of 9.6 million persons. Cancer data were obtained from the Swedish Cancer Registry from 1958 to 1996 and included 2,435 thyroid cancers among offspring. Seventy-eight families were identified in which a parent and an offspring had a thyroid cancer. The familial standardized incidence ratios (SIRs) were 7.8 and 2.5 for male and female adenocarcinomas (papillary and follicular cancer combined), giving a sex ratio of 2. 8. The familial SIRs for medullary and anaplastic carcinomas were about 4,000 and 300, respectively, without large sex difference. Medullary thyroid cancer has been coded as a separate entity since 1985, and the high familial SIR for anaplastic cancer was probably due to medullary cancer. The familial risks for all subgroups of thyroid cancer were highest in young age groups. The familial risk of medullary thyroid cancer may be the highest ever reported in population-based studies. Multiple endocrine neoplasia type 2 (MEN2) families were probably included but unambiguous diagnosis was not possible because of the coding practice. There was a strong association of medullary thyroid cancer in offspring and endocrine gland tumors in parents, which may be related to MEN2. Adenocarcinoma in offspring was not associated with discordant parental cancer.