Background: Spontaneous resolution of primary hyperparathyroidism (PHPT) is extremely rare, and has previously been reported exclusively in the setting of apoplexy (auto-infarction) of a large parathyroid adenoma outgrowing its blood supply. We report an unusual case of spontaneous PHPT resolution after Covid-19 infection. Clinical Case: A 38-year-old Hispanic woman was seen in clinic for evaluation of hypercalcemia first noted on routine bloodwork in 2018. She had a history of nephrolithiasis

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... 1 episode) but no other symptom associated with hypercalcemia. She was not on medications associated with hypercalcemia, and had no family history of Ca disorder. Serum biochemistry was notable for hypercalcemia (Ca: 10.9, 10.5, and 10.8 mg/dL, nl 8.4–10.2), normal albumin and Cr, elevated PTH (106 and 70 pg/mL, nl 15–65), with low phosphate (2.3 mg/dL, nl 2.4–4.5) and 25-OH-Vit. D (14 ng/mL, nl 20–80). 24-hr urinary studies showed elevated urine Ca (285 and 375 mg/day, nl 100–250), with urine Cr of 1.3 and 1.5 g/day, respectively. Findings of hypercalcemia, hypercalciuria, and elevated serum PTH, led to the diagnosis of PHPT. Neck ultrasound and sestamibi scan of the parathyroids with SPECT-CT did not localize any abnormal parathyroid gland. A month after PHPT diagnosis, and prior to surgical evaluation, patient presented with a 9-day history of fever, cough, and shortness of breath. She tested positive for Covid-19 infection by SARS-CoV-2 PCR on nasal swab, and was quarantined at home with symptomatic treatment. No glucocorticoids were given. Serum Ca was normal (9.5 mg/dL) at the time of Covid-19 diagnosis. All Covid-19 symptoms subsided 14 days after onset. Repeat labs 1 and 3 months after Covid-19 infection showed persistently normal serum Ca (10.0 and 9.8 mg/dL), with low then normal PTH (13 and 43 pg/mL), compatible with spontaneous resolution of PHPT. Patient denied any neck discomfort before, during, or after Covid-19 infection. Spontaneous resolution of PHPT is rare and follows apoplexy of a large parathyroid adenoma. In our patient, imaging failed to localize a large parathyroid adenoma, making it less likely that resolution of her PHPT was caused by apoplexy. Resolution of PHPT temporally coincided with Covid-19 infection, although the link between the two conditions is unclear at this time. Hypothesized mechanisms include an imbalance in the normal PTH-Ca axis caused by SARS-CoV-2 mediated release of inflammatory cytokines (e.g. interferon, previously reported to lower serum calcium), or development of antibodies against the parathyroid or CaSR. RNA and protein expression of ACE2, the SARS-CoV-2 cell receptor gene, is not detected in normal parathyroid tissue. Conclusion: To our knowledge, this is the first reported case of spontaneous resolution of PHPT after Covid-19 infection. Further studies are needed to understand the frequency of this occurrence, and the underlying mechanism.