A normal rate of thyroid hormone synthesis depends on an adequate dietary intake of iodine. Iodine is naturally present in water and soil, although some soils contain very low amounts. As a result, seafood is a more reliable source of iodine than crop plants. Approximately 1.6 billion people in more than 100 countries live in areas where natural sources of dietary iodine intake are marginal or insufficient. A minimum of 60 ^g of elemental iodine is required each day for thyroid hormone synthesis, and at least 100 ^g/day is required to eliminate thyroid follicular cell hyperplasia and thyroid enlargement (i.e., iodine deficiency goiter).
Subsequent to the ingestion of iodine in various forms, I" is absorbed by the small intestine and enters the blood. Two competing pathways are involved in the clearance of I" from the blood: renal filtration into urine and thyroidal uptake.The renal clearance rate for I" (30-50 mL/minute) varies only with the glomerular filtration rate. However, the thyroidal I" clearance rate is autoregulated to maintain an absolute thyroidal I" uptake rate of approximately 100 ^g I" each day. To accomplish this, the thyroidal I" clearance rate may vary (3 to 100 mL/minute) depending on the concentration of I" in the blood.
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