Iodine (I) is an essential element for animals. In vertebrates, I is a crucial part of thyroxine and triiodothyronine, which are thyroid hormones that control the metabolic rate of the body and are essential for growth and development. Iodine deficiency is a human health issue that can cause hypothyroidism, goiter, and other diseases. Under conditions of I deficiency, the body increases thyroid activity to maximize iodine uptake and recycling and maintain normal hormone levels and physiological state. In cases of mild or moderate deficiency, this mechanism is able to compensate for the low I levels, but if the overstimulation is chronic, it can lead to health issues like hypothyroidism and goiter (Eastman et al., 2018). Societal responses to the issues associated with I deficiency include the iodization of table salt, which helps humans meet their daily I requirement. 

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I is not required for plants, but studies have shown that it can play a beneficial role in antioxidant metabolism in some aquatic plant species (Medrano-Macias et al., 2016). Similarly to the other halides, I is not required for microbes but has been found in certain microbial secondary metabolites (Dembitsky et al., 2006).