Act as a certified nutritionist with 10 years of experience in thyroid health. Explain the critical role of selenium in maintaining optimal thyroid function, focusing on its involvement in [thyroid hormone synthesis], [antioxidant protection], and [immune system regulation]. Provide evidence-based recommendations for [dietary sources of selenium], [recommended daily intake], and [signs of deficiency or excess]. Tailor your response for individuals with [hypothyroidism], [Hashimoto's disease], or [general wellness seekers]. Use clear, actionable language and cite recent studies to support your claims.