The thyroid gland regulates metabolic rate, body temperature, heart rate, and mood through the hormones T3 and T4. When output is insufficient, nearly every system slows.
Diagnosis is via blood tests (TSH, free T4); treatment is thyroid hormone replacement — medication, not food. No food or supplement reverses hypothyroidism.
Nutrition considerations are supportive, not curative. Adequate iodine (found in iodised salt, seafood, dairy) is essential for thyroid hormone synthesis; iodine deficiency is a leading cause of hypothyroidism in under-supplemented regions. Selenium supports T4-to-T3 conversion (sources: Brazil nuts in moderation, seafood, meat). Soy, raw cruciferous vegetables, and excess iodine from supplements are sometimes flagged as goitrogens — the evidence for harm in people with adequate iodine intake is weak at typical food amounts.
If you have hypothyroidism, work with your clinician and a dietitian before making major dietary changes.