Usually you as an individual won't know how much iodine you are getting, particularly in countries like the United States, because iodine appears in commercial preparations without notice. You can make a rough calculation, based on (1) whether you use iodized salt; (2) how much salt you eat; (3) whether you take vitamins that contain iodine (many contain 150 mcg); and (4) whether you eat much meat, dairy products, or seafood.

For populations, a better way to learn the iodine intake is to measure the amount of iodine in representative urine samples. Most (more than 90%) of the iodine you ingest eventually comes out in the urine. Thus, the concentration of iodine in the urine, even in casual samples, is a good marker for iodine nutrition. Urine iodine concentration varies with fluid intake, so the established ranges have limited use for casual samples from an individual, but they are good for assessing a population group, because individual variations tend to average out. A median urinary iodine concentration between 100 and 200 mcg/L is ideal.

Routine laboratory tests of thyroid function are not as helpful as the urinary iodine concentration in assessing a population. The serum TSH (thyroid stimulating hormone or thyrotropin) is a test that is widely used clinically to assess thyroid function in individuals. TSH is released into the blood stream by the pituitary (an endocrine control gland at the base of the brain) in response to the amount of thyroid hormone in the blood stream. TSH tells the thyroid how hard to work. It bases this decision on how much thyroid hormone is present in the blood. When blood thyroid hormone is low, serum TSH increases and this can be used as a measure for inadequate thyroid hormone production. The serum TSH is the most widely used test for diagnosing thyroid disease in individuals. However, the serum TSH is less valuable in recognizing iodine deficiency, because it can increase but still remain within the normal range.

An exception is the blood TSH in newborns. Most developed countries have a system of newborn screening, either with the blood TSH or thyroxine, to recognize congenital hypothyroidism. About one out of 4,000 newborns has congenital deficiency of thyroid hormone production, usually because the thyroid is absent or fails to develop properly. This newborn screening is important because prompt treatment with thyroid hormone can prevent most or all of the consequences of inadequate thyroid hormone on brain development. Newborn TSH screening in areas with iodine deficiency shows an increase in the number of infants with transient hypothyroidism. Usually, this transient hypothyroidism corrects itself, and most of these infants do not show brain damage, but there remains a potential risk for this result, particularly if they continue iodine deficient during nursing.

Thyroid size is also a useful marker of iodine nutrition, because the iodine-deficient thyroid enlarges, as described above. One way to recognize goiter is to feel the thyroid and make an estimate of its size; this maneuver is quite simple but fairly crude and not very accurate in detecting slight enlargement, particularly in children. Use of an ultrasound machine is simple and can be done in remote settings; this measurement provides another good index of the degree of iodine deficiency.