Can vitamin D (Vitamin D) insufficiency cause suppressed Thyroid-Stimulating Hormone (TSH) levels?

Vitamin D Insufficiency Does Not Cause Suppressed TSH Levels

Vitamin D insufficiency does not cause suppressed TSH levels; in fact, the opposite relationship exists—low vitamin D is associated with elevated TSH, not suppressed TSH. The evidence consistently demonstrates that vitamin D deficiency correlates with higher TSH levels and increased risk of hypothyroidism, not hyperthyroidism or TSH suppression.

Understanding the Relationship Between Vitamin D and TSH

The physiological relationship between vitamin D and thyroid function shows that:

• Low vitamin D levels are independently associated with high serum TSH levels, not suppressed TSH 1.
• A significant negative correlation exists between serum 25(OH)D and TSH levels (r = -0.127, p = 0.013), meaning as vitamin D decreases, TSH increases 1.
• Patients with overt hypothyroidism (characterized by elevated TSH) have significantly higher prevalence of vitamin D insufficiency (60.4%) and lower 25(OH)D levels compared to euthyroid patients 1.

What Actually Causes Suppressed TSH

Suppressed TSH levels (typically <0.1 mU/L) are caused by:

• Excessive thyroid hormone production (hyperthyroidism from Graves' disease, toxic nodular goiter, thyroiditis) 2.
• Excessive levothyroxine therapy in patients being treated for thyroid cancer or hypothyroidism 2.
• Intentional TSH suppression as part of thyroid cancer management, where TSH levels are maintained below 0.1 mU/L in high-risk patients 2.

Clinical Context: Vitamin D Toxicity and PTH Suppression

There is an important distinction to make regarding suppressed hormone levels:

• Vitamin D toxicity (not insufficiency) causes suppressed parathyroid hormone (PTH), not suppressed TSH 3.
• Suppressed PTH levels occur secondary to excessive vitamin D therapy, typically at 25(OH)D levels >150 ng/mL 2, 3.
• This PTH suppression is completely separate from thyroid function and TSH regulation 2.

Evidence from Mendelian Randomization Studies

The most robust causal evidence comes from genetic studies:

• Each 1 SD increase in serum 25(OH)D concentration is associated with a 12% decrease in the risk of high TSH (p = 0.02), confirming that higher vitamin D protects against elevated TSH, not the reverse 4.
• Higher genetically predicted vitamin D concentration lowers the odds of having high TSH or autoimmune hypothyroidism by 16.34% (p = 0.02) 4.
• No causal relationship was found between vitamin D and low TSH or hyperthyroidism 4.

Effect of Vitamin D Supplementation on Thyroid Function

When vitamin D supplementation is provided to deficient patients:

• TSH levels decrease significantly with vitamin D supplementation (50,000 IU weekly for 3 months), not increase 5.
• Anti-thyroglobulin antibody levels and TSH both showed significant reduction in the vitamin D group compared to baseline 5.
• No significant changes occur in T3 or T4 levels with vitamin D supplementation 5.

Clinical Implications and Common Pitfalls

The critical pitfall is confusing the direction of association: vitamin D insufficiency is linked to elevated TSH (hypothyroidism), not suppressed TSH (hyperthyroidism). When evaluating a patient with suppressed TSH:

• Do not attribute suppressed TSH to vitamin D insufficiency—look for hyperthyroidism, excessive thyroid hormone replacement, or other causes of thyrotoxicosis 2.
• Vitamin D deficiency should be evaluated in patients with elevated TSH and autoimmune hypothyroidism, where supplementation may provide benefit 1, 5, 4.
• The association between vitamin D and thyroid function is most pronounced in autoimmune thyroid disease, particularly Hashimoto's thyroiditis with overt hypothyroidism 1, 6.

Nuances in the Evidence

While most studies show negative correlations between vitamin D and TSH:

• Some studies in healthy participants have observed either negative correlation or no association between TSH and 25(OH)D levels 6.
• One study found no association between Hashimoto's thyroiditis and vitamin D deficiency, though this study had methodological limitations 7.
• The variability between studies may be due to different assays, confounding factors (sex, age, BMI, season), and population differences 6.