Differentiation of postpartum Graves' thyrotoxicosis from postpartum destructive thyrotoxicosis using antithyrotropin receptor antibodies and thyroid blood flow.

Background: Postpartum thyroid dysfunction occurs in approximately 5-10% of women in the general population within one year of delivery. Differentiation of postpartum Graves' thyrotoxicosis (PPGr) from postpartum destructive thyrotoxicosis (PPDT) is essential because of the difference in treatment measures between the two. However, it is sometimes difficult because radioactive iodine uptake is contraindicated when patients are lactating. We examined the usefulness of determining the time of onset postpartum and measurement of antithyrotropin (anti-TSH) receptor antibodies and thyroid blood flow. Methods: Forty-two patients with newly developed thyrotoxicosis after delivery were examined: 18 had Graves' disease and 24 had destructive thyrotoxicosis. Serum free thyroxine (fT4), free triiodothyronine (fT3), and TSH were measured by chemiluminescent immunoassays. Anti-TSH receptor antibodies (TRAb), antithyroglobulin antibodies (TgAb), and antithyroid peroxidase antibodies (TPOAb) were measured by the Elecsys electrochemiluminescence immunoassay. Thyroid volume and blood flow (TBF) were measured quantitatively by color flow Doppler ultrasonography. Results: Onset of thyrotoxicosis was distributed from 2 to 12 months postpartum. Twelve (85.7%) of 14 patients who developed thyrotoxicosis at three months or earlier after delivery had PPDT. On the other hand, all 11 patients who developed thyrotoxicosis at 6.5 months or later had PPGr. All patients with PPGr had positive TRAb (14.9 +/- 14.9 IU/L, mean +/- standard deviation (SD)) and all patients with PPDT had negative TRAb (0.1 +/- 0.3 IU/L, p < 0.0001). Fifteen (83.3%) of 18 PPGr patients had high TBF of more than 4.0% (8.9 +/- 4.4), and all PPDT patients had low TBF of < 4.0% (1.6 +/- 1.0, p < 0.0001). The fT3/fT4 ratio was higher in PPGr (64.0 +/- 23.9) than in PPDT (38.9 +/- 13.1, p < 0.0002), but absolute values overlapped between the two. Conclusion: Early onset of thyrotoxicosis postpartum was associated mainly with PPDT, and a late onset was suggestive of PPGr. Positive TRAb and high TBF >4.0% are indicators of postpartum onset of Graves' disease.