Case Report: Resolution of complete heart block following vitamin D supplementation in a child with Graves disease.

An 11-year-old male with Graves disease presented with complete heart block (CHB) that had progressed from a pre-existing Mobitz type 1 second-degree AV block one month prior to Graves disease diagnosis.

• Patient was 9 years old at presentation in January 2023.
• Presenting symptoms included weight loss, heat intolerance, and excessive sweating.
• Laboratory values at presentation: TSH <0.01 uIU/mL, free thyroxine (FT4) 4.8 ng/dL, total thyroxine (T4) 16.7 µg/dL, and total triiodothyronine (T3) 334 ng/dL.
• The CHB persisted despite the patient becoming biochemically euthyroid within one month of methimazole treatment.

Complete heart block persisted after achievement of euthyroid state on methimazole, prompting a cardiologist recommendation for pacemaker implantation by April 2023.

• Patient achieved biochemical euthyroid status within one month of methimazole initiation.
• Despite euthyroid status, CHB continued unresolved.
• By April 2023, the cardiologist recommended pacemaker implantation.
• The family declined pacemaker implantation and opted for continued monitoring instead.

Vitamin D3 supplementation at 2500 IU daily was initiated in late October 2023 following the mother's independent research into potential benefits of vitamin D in autoimmune thyroid disease.

• Supplementation was started in late October 2023, approximately 9 months after Graves disease diagnosis.
• No additional medications were used to treat the cardiac condition.
• The decision to supplement was initiated by the family, not the treating physicians.
• The dose was subsequently reduced to 2000 IU daily during continued follow-up.

Nighttime bradycardia resolved three months after initiating vitamin D3 supplementation, and a 12-lead ECG in April 2024 confirmed resolution of CHB to first-degree AV block.

• Nighttime bradycardia resolved approximately three months after starting vitamin D3 (around January/February 2024).
• A 12-lead ECG in April 2024 confirmed resolution of complete heart block to first-degree AV block.
• No additional cardiac medications were used during this period.
• The patient remained stable with first-degree heart block while euthyroid on low-dose methimazole.

Peak TSH receptor antibody (TSHRab) and thyroid-stimulating immunoglobulin (TSI) values decreased following initiation of vitamin D3 supplementation.

• Both TSHRab and TSI values declined after vitamin D3 supplementation was started.
• The paper does not provide specific numerical values for TSHRab and TSI before and after supplementation in the abstract.
• This decrease was observed in the context of continued low-dose methimazole use.
• The authors propose a potential synergistic effect between vitamin D and methimazole in achieving better thyroid control.

The authors proposed several mechanisms to explain the observed association between vitamin D supplementation and AV block improvement, including immunomodulatory effects, cardioprotective properties, and synergy with methimazole.

• Proposed mechanisms include vitamin D's immunomodulatory effects on Graves disease.
• Vitamin D's cardioprotective properties were identified as a potential contributing mechanism.
• A potential synergistic effect of vitamin D with methimazole in achieving better thyroid control was proposed.
• The authors note these are proposed mechanisms and that further investigation is warranted.