The “worst” low-iron symptoms often involve functional impairment and cardiopulmonary strain: shortness of breath with minimal effort, palpitations even at rest, profound fatigue, near-fainting, chest tightness, and major performance decline. Some people also experience restless legs, severe dizziness, and significant concentration problems. If symptoms are severe or rapidly worsening, prompt clinical evaluation is important because ongoing blood loss or other medical conditions may be involved.