**2.5.2 Plasma**

The inorganic form of iodine represents about 0.5 % of the total plasma iodine. The rest occurs in bound form with specific plasma protein (protein - bound iodine, PBI) which has gained wide use as an indicator of thyroid activity in humans. It has been reported that the total plasma iodine concentration in healthy subjects is between 40 and 80 μg/l. According to Allain's studies when plasma iodine concentrations are below 40 μg/l, hypothyroidism is highly likely, when they are between 80 and 250 μg/l, hyperthyroidism, particularly Graves' disease is probable. Above 250 μg/l – iodine overload is almost certainly indicated (Allain et al., 1993).
