Iodine in biology
The U.S. Food and Nutrition Board and Institute of Medicine recommended daily allowance of iodine ranges from 150 micrograms per day for adult humans to 290 micrograms per day for lactating mothers. However, the thyroid gland needs no more than 70 micrograms per day to synthesize the requisite daily amounts of T4 and T3. The higher recommended daily allowance levels of iodine seem necessary for optimal function of a number of other body systems, including lactating breasts, gastric mucosa, salivary glands, oral mucosa, arterial walls, thymus, epidermis, choroid plexus and cerebrospinal fluid, among others.
Iodine-125
Iodine-125 is commercially available in dilute NaOH solution as I-iodide (or the hypohalite sodium hypoiodite, NaOI) . The radioactive concentration lies at 4 to 11 GBq/ml and the specific radioactivity is >75GBq/µmol. The chemical and radiochemical purity is high. The radionuclidic purity is also high; some I (t=13.1d) is unavoidable due to the neutron capture noted above. The I-126 tolerable content (which is set by the unwanted isotope interfering with dose calculations in brachytherapy) lies at about 0.2% atom fraction of the total iodine (the rest being I-125).