Thyroid follicular cells secrete thyroxine (T4) and triiodothyronine (T3); T3 is more biologically active though T4 is secreted in larger amount and acts mainly as a prohormone.
Normal total plasma concentration: T4 about 8 μg/dL, T3 about 0.15 μg/dL.
Free hormone: free T4 is about 2 ng/dL and free T3 about 0.3 ng/dL.
Iodine Metabolism
Iodine and tyrosine are the raw materials for thyroid hormone synthesis; daily ingestion of 150 μg of iodine maintains normal thyroid function.
Daily turnover: about 500 μg of iodine is ingested, of which 120 μg/day is taken up by the thyroid; normal plasma iodide is 0.3 μg/dL.
Kidney is the principal excretory organ, excreting about 480 μg/day in urine; the thyroid secretes 80 μg/day of iodine as T3 and T4 and 40 μg/day directly into ECF.
Liver loses about 20 μg/day of iodine in bile and stool.