The role of thyroid stimulating antibody (TSAb) in the thyroid function of patients with post-partum hypothyroidism.

OBJECTIVE

We investigated the association between thyroid function and the biological activities of thyroid stimulating antibodies (TSAb) and thyroid stimulation blocking antibodies (TSBAb) in patients with post-partum hypothyroidism.

DESIGN

A prospective study.

PATIENTS

We studied 25 patients with post-partum hypothyroidism who visited our thyroid clinic during the period from 1985 to 1990.

MEASUREMENTS

We measured TSH binding inhibitory immunoglobulin (TBII) and TSAb activity at the initial presentation of each of the 25 patients. Women found to have elevated TSAb activity were followed up. Upon finding negative TSAb activity along with positive TBII activity in the serum at the initial presentation, we measured TSBAb activity. Women found to have elevated levels of TSBAb at the initial presentation were also followed up.

RESULTS

Elevated TBII activity was found in six of the 25 patients, as was high TSAb activity (205-2651%, normal 55.0-145.0%) in five of these six and in one other patient at the initial presentation. Markedly elevated TSBAb activity (89%) was found in one TBII positive patient. We were able to follow up serially five TSAb positive patients and the TSBAb positive patient over periods ranging from 11.5 to 26.5 months post-partum. The maximal value of TSAb activity was observed at the initial presentation in all TSAb positive patients, following which the activities gradually decreased. One of these patients developed Graves' hyperthyroidism associated with high TSAb activity (1223%) at 10.5 months post-partum. One of the other patients was restored to euthyroid with elevated TSAb activity (279%), but thereafter developed hypothyroidism in conjunction with the disappearance of TSAb activity at 26.5 months post-partum. In the other two patients, normalization of thyroid function was observed with elevated TSAb activity. Thereafter, thyroid function remained within the normal range even with the disappearance of TSAb activity. In the other patient, normalization of thyroid function was observed at 11.5 months post-partum, 3 months after the disappearance of TSAb activity. In the TSBAb positive patient, TSBAb activity decreased to 21% by 17.5 months post-partum associated with normalization of thyroid function.

CONCLUSION

The present study demonstrates the presence of elevated levels of TSAb activity in some patients with post-partum hypothyroidism. In these patients, Graves' hyperthyroidism may be induced by TSAb activity, and hypothyroidism may reoccur with the disappearance of the TSAb activity. Furthermore, post-partum hypothyroidism may be due to increased TSBAb activity in some patients.