How low are iron levels considered to be that cause side effects?

Iron levels below 30 micrograms per deciliter (mcg/dL) in serum or a ferritin level below 30 nanograms per milliliter (ng/mL) are generally considered indicative of iron deficiency and may lead to side effects such as fatigue, weakness, and impaired cognitive function. Symptoms can vary based on the severity of the deficiency, and some individuals may experience side effects at higher levels if other underlying health issues are present. It's important to consult a healthcare provider for proper diagnosis and management.