SAT-322 - A Case of Autoimmune Thyroid Disease with Oscillation between Hypo- and Hyperthyroidism and Concomitant Anti-TPO and Trab Positivity, Exploring Destructive Thyroiditis Versus Functional Switching of TSH Receptor Antibodies.

Background
Autoimmune thyroid disease usually describes as either stable hypo- or hyperthyroidism. This can lead to fluctuations in thyroid function in a minority of patients, creating diagnostic and therapeutic difficulties. Anti-thyroid peroxidase antibodies (anti-TPO) and TSH receptor antibodies (TRAb) coexist, either by overlapping destructive thyroiditis with Graves’ disease or functional heterogeneity of TRAb and may explain such oscillations.

Case Presentation
In this case a patient with the last 10 years of autoimmune thyroid disease with persistent hypothyroid to hyperthyroid changes. The patient needed periodic replacement of levothyroxine during hypothyroidism and of antithyroid therapy for episodes of hyperthyroidism. Tests carried out multiple times showed unremitting anti-TPO and TRAb positivity. Thyroid ultrasound revealed diffusely inhomogeneous parenchyma without nodular disease, supporting chronic autoimmune thyroiditis. The history of thyroid scintigraphy in a hyperthyroid phase had shown homogeneously increased uptake consistent with Graves’ disease. Most recently, the patient presented with subclinical hyperthyroidism with biochemical and clinical signs of relapse of Graves’ disease and responded to antithyroid treatment.

Discussion
This case presents two main mechanistic explanations. The first is presence of Hashimoto’s thyroiditis and Graves’ disease, alternating dominance of destructive and stimulatory autoimmune mechanisms. The second, possibly unifying mechanism is a functional switch of TRAb, where changes in stimulating and blocking antibody activity ultimately modulate functioning of the thyroid over time. Standard TRAb binding assays confirm antibody presence, but they do not differentiate functional activity, which limits the interpretation of variable phenotypes. The recognition of functional TRAb dynamics has crucial implications for diagnosis, monitoring and therapeutic decision-making.

Conclusion
A rare but clinically meaningful presentation: oscillating autoimmune thyroid disease. In patients with alternating hypo- and hyperthyroidism and concomitant anti-TPO and TRAb positivity, functional TRAb switching should be considered. A mechanism-based approach could expedite evaluation for definitive therapeutic intervention and better long-term treatment options.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*