Thyroid supplementation for hypothyroidism. An latrogenic cause of breast cancer?

In four subpopulations grouped by varying durations of thyroid supplementation (none, one to five years, six to 15 years, and 15 years of more), breast cancer frequency correlated positively with attained age. The cancer experience of persons in the same age-group was relatively stable regardless of how long women took thyroid supplements. The present data support the provisional inference that breast cancer and duration of thyroid therapy are both age-dependent and question the belief that pretreatment with thyroid supplements increases the risk of breast cancer development. Relationship of these two variables can only be determined by prospective studies.