A new registry-based study from Sweden suggests men with severe infertility may face increased risks of colorectal and thyroid cancers. Researchers examined data on more than 1.1 million men who fathered children between 1994 and 2014. Among them, 14,450 men became fathers using intracytoplasmic sperm injection (ICSI), an assisted reproduction technique commonly used for severe male infertility. By linking fertility and national cancer registries, the team found a statistically significant elevation in colorectal and thyroid cancer diagnoses in the group of fathers who had used ICSI compared with other fathers.
The study’s authors and outside experts emphasize that this is an association, not proof that ICSI or assisted reproduction causes cancer. Several explanations are possible. One is shared underlying biology: thousands of genes influence reproductive function, and some genetic alterations could both impair fertility and raise cancer risk. Lifestyle and metabolic factors that reduce semen quality—such as smoking, heavy alcohol use, obesity, and inactivity—also increase cancer risk. The Swedish analysis adjusted for parental age and educational level but lacked individual lifestyle data, so the contribution of these factors remains uncertain.
The investigators highlight that assisted reproductive techniques themselves are not shown to cause cancer. Instead, severe male infertility could mark a subgroup of men who would benefit from greater medical follow-up and preventive care. Study authors recommend considering earlier or targeted screening for colorectal, thyroid, and testicular cancers during fertility evaluations and follow-up, especially since colorectal and thyroid cancer rates are rising in younger adults. Clinicians continue to advise standard cancer-risk–reducing behaviors: maintain a healthy weight, limit alcohol, avoid tobacco, stay physically active, and follow routine screening guidelines.
In summary, severe male infertility—proxied in this study by use of ICSI—was linked to higher colorectal and thyroid cancer risk in this large Swedish cohort. The finding calls for further research into genetic and lifestyle contributors and for consideration of tailored follow-up for men with severe fertility problems, while remembering that association does not equal causation.