The influence of pregnancy and reproductive span on the occurrence of autoimmune thyroiditis.

A study comparing the reproductive histories of 89 women affected by Hashimoto's thyroiditis (HT) with 89 matched controls has been carried out to determine whether endogenous hormones contribute to the development of the disease. There was no evidence of an association between HT and parity and no trend to an increasing disease risk with increasing parity. However, patients with HT had a longer reproductive span. There was a significant trend to a lower risk with later age at menarche (relative risk (RR) of menarche at 13-14 years compared with less than or equal to 12 years = 0.71, and greater than or equal to 15 years compared with less than or equal to 12 years = 0.45), and an increased risk with a later menopause (greater than or equal to 51 years compared with less than or equal to 50 years, RR = 3.00, P less than 0.025). An additional finding was an increased hysterectomy rate among the cases. These observations may be explained if the hyperoestrogenic state associated with a longer reproductive span acted to modulate the immune process in autoimmune thyroid disease.