The physiologic importance of iodine rests in its essentiality for the formation of the thyroid hormone which maintains control of energy metabolism through some unknown pathway. The human requirement for iodine is about 1 g. per kilogram of body weight daily for basal needs, and an additional 2 pg/kg. for the maintenance of total metabolic activity. The suggested daily allowance for an adult is 0.15 to 0.30 mg. daily (0.002 to 0.004 mg/kg. body weight) (4a). This amount can be supplied by the regular use of iodized salt.
Iodine is absorbed from the small intestine at an exponential rate which varies with the level of thyroid activity . Iodine is taken up by the thyroid gland which converts it to di-iodotyrosine and thyroxine. In normal persons, the iodine uptake is proportional to blood concentration. Measurement of the uptake of radioactive iodine is a useful procedure in the diagnosis of thyroid disease; uptake is increased in hyperthyroidism and de-creased in hypothyroidism altho numerous factors can influence findings.
The iodine in blood is in two forms, inorganic and protein-bound iodine, the later presumably representing the circulating thyroid hormone. The total iodine of whole blood ranges from 3 to 30 ug/100 ml., usually 8-12 pg, while protein bound iodine varies from 3 to 8 pg., mean 5-6 pg. The level of protein-bound iodine is decreased in hypothyroidism and increased in hyperthyroidism and during pregnancy. Urinary excretion of iodine is dependent upon dietary intake, the activity of the thyroid gland and probably other factors.
Iodine deficiency leads to the development of endemic goiter. In areas of the world where the iodine content of the water, soil, and foods grown therein are low, the incidence of endemic goiter is high. Goiter can be produced by factors other than iodine deficiency. Goitrogenic agents have been found in foods, especially in those of the cabbage family, and a number of synthetic chemical compounds have goitrogenic activity. In endemic goiter, the thyroid gland may contain less than 1 mg. of iodine whereas the normal thyroid contains 8 to 10 mg.
In areas where endemic goiter is prevalent, congenital hypothyroidism or cretinism is observed. In cretinism, growth may be stimulated and normal mental and physical development attained by continuous administration of thyroid hormone. This indicates that iodine is essential for growth; it is also essential for normal fertility and for lactation.
Diagnosis of endemic goiter is suggested by the presence of an enlarged thyroid gland in a person living in an area where iodine intake is low. Other causes of hyper-trophy of the thyroid must be ruled out. Thyroid enlargement is most commonly seen during puberty and in pregnancy when the need for iodine is increased. In goitrogenous areas, the urinary excretion of iodine is low, ranging from less than 10 to about 65 ,ug. daily, as compared to 72 to 343 g. daily in nongoitrogenous areas. The protein-bound iodine level in blood is usually within the normal range in persons with endemic goiter. The uptake of I”‘ by the thyroid gland of these individuals is higher than that of euthyroid persons in other areas of the world. Hence, this test is not useful in the diagnosis of hyperthyroidism in regions where endemic goiter is prevalent.