| Summary: | ABSTRACT: Objective: To report the case of a female patient with hypothyroidism who spontaneously developed Graves hyperthyroidism during pregnancy and then reverted back to hypothyroidism in a subsequent pregnancy.Methods: The pertinent clinical features, laboratory data, and clinical course of the case are reported, along with a brief literature review.Results: A 30-year-old female with hypothyroidism diagnosed at age 20 years unexpectedly required decreased levothyroxine dosing during her second pregnancy. She was taking levothyroxine 12.5 μg daily when she became pregnant a third time. In the first trimester, levothyroxine was discontinued, and she presented in the third trimester with clinical and biochemical hyperthyroidism and a diffusely enlarged goiter. Propylthiouracil (PTU) was initiated. Thyroid-stimulating hormone–receptor antibodies (TRAbs) were positive. After delivery, her baby developed transient neonatal Graves disease. She was continued on a stable dose of PTU for 10 months postpartum and then became pregnant a fourth time. PTU was discontinued and she remained euthyroid off medications until the second trimester, when she presented with clinical and biochemical hypothyroidism, requiring levothyroxine initiation. TRAb level was elevated. Thyroid-stimulating immunoglobulin (TSI) bioassay was elevated. Despite elevated TSI in her fourth pregnancy, her child did not develop neonatal Graves disease. She remains euthyroid on levothyroxine.Conclusion: Spontaneous transformation from hypothyroidism to hyperthyroidism during pregnancy is rare but can occur. The balance between the activity of stimulating and blocking TRAbs may impact the clinical presentation for both the mother and the fetus.Abbreviations: AITD autoimmune thyroid disease; FT3 free triiodothyronine; FT4 free thyroxine; GD Graves disease; LT4 levothyroxine; PTU propylthiouracil; TBAb thyroid-blocking antibody; TRAb thyroid-stimulating hormone–receptor antibody; TSAb thyroid-stimulating antibody; TSH thyroid-stimulating hormone
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