Use of plasma iodine assay for diagnosing thyroid disorders

To examine the advantage of systematic plasma iodine assays in establishing the thyroid function of patients with thyroid disorders.

Iodine was determined by inductively coupled plasma mass spectrometry (ICPMS) in the plasma of 799 patients consulting for possible thyroid disorders, indicated by FT4 and TSH assays.

Plasma iodine was below 40 micrograms/l in 57 (7%) patients, most of whom had hypothyroidism; 40-80 micrograms/l in 439 (55%) patients, most of whom had normal thyroid hormone function; 80-250 micrograms/l in 240 (30%) patients, most of whom had hyperthyroidism; and above 250 micrograms/l in 63 (8%) patients, almost all of whom had iodine overload caused by iodinated drugs, particularly amiodarone, resulting in euthyroidism (24%), hyperthyroidism (36%), and hypothyroidism (16%). Sixty five (7%) had been treated with amiodarone and 27 (3%) with other iodinated drugs. More than 10% of patients with thyroid disorders therefore had an iodine overload.

The determination of total plasma iodine using the simple, accurate ICPMS technique, should be carried out in patients consulting for thyroid disorders, particularly for the detection of an iodine overload.