Introduction:
Papillary thyroid carcinoma has a well-recognized familial pattern. However, the specific risk to close and extended relatives of patients diagnosed with papillary thyroid carcinoma has yet to be adequately defined.

Methods:
Using the Utah Population Database, an extensive genealogical database linked to medical records and the Utah Cancer Registry, papillary thyroid carcinoma risk was calculated for 1st through 5th degree relatives and spouses of probands compared to random population-based controls matched 5:1 on sex, year of birth, and place of birth. Familial risk was estimated by calculating odds ratios using conditional logistic regression, adjusting for number of biological relatives, their degree of relatedness, and their person-years at risk. All relatives of pediatric cases and of matched controls with follow-up who linked to a pedigree of ≥2 generations were included. This approach has been shown to lead to unbiased familial risk estimates. As observations within families are non-independent, a robust variance estimator for cluster-correlated data was incorporated.

Results:
First-, second-, and third-degree relatives of 4,460 papillary thyroid carcinoma probands diagnosed from 1966-2011 had a significant increased risk of developing this malignancy compared to population controls. First-degree relatives of probands were at 5.4-fold increased risk (P<10-15) of being diagnosed with this cancer themselves. Second- and third-degree relatives had a 2.2-fold (P<10-11) and 1.8-fold risk (P<10-8), respectively, of developing papillary thyroid carcinoma. Siblings of probands were at highest risk (OR=6.8, P<10-15). There was no significant increased risk observed in spouses of probands.

Conclusion:
In the largest population study to date, a high risk of papillary thyroid carcinoma is confirmed in first degree relatives. Furthermore, significant risk extends to second- and third-degree relatives but not to spouses of probands, suggesting that this risk may have a genetic basis rather than shared environment. These findings indicate that this group will likely benefit from closer clinical attention, including collecting a 3-generation family history. Studies are needed to better define optimal screening approaches and implementation.