Relationship between Iron Deficiency and Thyroid Function: A Systematic Review and Meta-Analysis.
Level 4 - case-series / case-control
Systematic review and meta-analysis consisting entirely of cross-sectional observational studies.
PubMed 38004184 · doi:10.3390/nu15224790
What was done
A systematic review and meta-analysis was conducted to evaluate differences in thyroid stimulating hormone (TSH), free thyroxine (FT4), and free triiodothyronine (FT3) between individuals with iron deficiency (ID) and healthy non-ID controls. Study quality was evaluated using Cambridge Quality Checklists. Analyses included calculating mean differences (MD), conducting subgroup analyses across pregnant and non-pregnant women, running meta-regression between serum ferritin and TSH/FT4, and reviewing thyroid autoantibody prevalence.
What was found
Ten cross-sectional studies were included. Patients with ID had significantly lower hormone levels compared to controls: - TSH: MD -0.24 mIU/L (95% CI -0.41 to -0.07; I² = 100%, p = 0.005) - FT4: MD -1.18 pmol/L (95% CI -1.43 to -0.94; I² = 99%, p < 0.000001) - FT3: MD -0.22 pmol/L (95% CI -0.32 to -0.12; I² = 99%, p < 0.00001) Subgroup analysis confirmed lower TSH, FT4, and FT3 in pregnant women with ID. In non-pregnant women with ID, FT4 and FT3 were lower, but TSH did not differ. Meta-regression found positive correlations between serum ferritin and both TSH and FT4. ID was also associated with increased positivity for anti-thyroglobulin and anti-thyroid peroxidase antibodies.
Why it matters
The findings synthesize evidence linking iron deficiency to lower circulating thyroid hormone levels and thyroid autoimmunity, highlighting the potential role of iron status in thyroid hormone homeostasis, especially during pregnancy.
Limits
All included studies were cross-sectional, preventing causal conclusions. Statistical heterogeneity was severe across all primary meta-analyses (I² = 99–100%). Total participant sample sizes, specific diagnostic thresholds for iron deficiency, and control for key confounders (such as iodine status or inflammation) were not reported in the abstract.
Cited by
- supports Adequate iron levels are required for proper thyroid function and thyroid hormone synthesis.