The search strategy used a combination of relevant medical subject headings (MeSH terms) and keywords, including (“beta carotene” OR “beta-Carotene 15,15-Monooxygenase”) AND (“Thyroid Hormones” OR “hypothyroidism” OR “hyperthyroidism” OR “T3 thyroid hormone” OR “T4 thyroid hormones” OR “mechanism of action” OR “thyroid” OR “vitamin A” OR “Retinoid” OR “Retinol” OR “thyroid stimulating hormone” OR “TSH”). This evidence concerns the gene TG and hypothyroidism.