Vitamin D and Thyroid Disease
Vitamin D acts as an immunomodulator, biasing T-cell responses away from Th1/Th17 autoimmunity. Because Hashimoto's and Graves' disease are the two most common autoimmune diseases in adults, vitamin D's relevance to thyroid health is more than theoretical.
Hashimoto's thyroiditis
Meta-analyses consistently show serum 25(OH)D is 30–40% lower in Hashimoto's patients than in matched controls, and low 25(OH)D correlates with higher TPO (thyroid peroxidase) antibody titres. Whether that's cause, consequence, or shared autoimmune-susceptibility genotype has been hard to pin down.
Randomised supplementation trials in Hashimoto's patients have shown modest reductions in TPO antibodies (typically 10–30% over 3–6 months of 2,000–4,000 IU/day cholecalciferol), particularly in patients with baseline deficiency. Whether this translates to slower progression to overt hypothyroidism is not yet established, but the biological plausibility plus the low cost make repletion sensible.
Graves' disease
Similar observational patterns: lower 25(OH)D in Graves' patients versus controls. Small RCTs suggest correcting deficiency may reduce relapse rates after antithyroid medication is stopped, though the trials are underpowered. Vitamin D also protects bone during hyperthyroidism, when accelerated bone turnover raises osteoporosis risk.
Effect on TSH and thyroid hormone levels
In patients already on levothyroxine, vitamin D correction has minimal effect on TSH or free T4 — the dose is titrated to those levels directly. If your TSH is unstable and you recently started high-dose vitamin D, an unrelated absorption change may be the reason (calcium and iron supplements taken with levothyroxine are more common culprits).
Practical dosing for thyroid patients
- Test serum 25(OH)D at diagnosis of any autoimmune thyroid disease.
- Target 30–50 ng/mL. Typical dose 1,500–4,000 IU/day cholecalciferol depending on baseline and body weight.
- Retest at 3 months.
- Separate vitamin D from levothyroxine by at least 4 hours — no known interaction, but standard best-practice for other hormone therapy overlap.
- Combined with selenium 200 µg/day (also linked to lower TPO antibodies in Hashimoto's) if desired, though the evidence is similarly modest.