Which of the following statements is true about the synthesis of thyroid hormone and its physiology?

A. The iodine utilized in hormone synthesis is derived principally from dietary sources.

C. Enough thyroxine (T 4) is stored in the normal thyroid to provide a euthyroid state for 3 weeks despite absence of iodine intake.

DISCUSSION: Iodine is necessary for the synthesis of thyroid hormone, and approximately 200 to 500 mg is ingested daily. Most of it is absorbed from the small intestine and is cleared from the plasma by secretion of thyroid hormone. TSH is required for the normal production and secretion of thyroid hormone. It is generally accepted that TSH also has a major role in thyroid growth. The thyroid gland has a storage reserve of approximately 3 weeks. The principal regulatory mechanisms of the thyroid gland are the hypothalamic-pituitary-thyroid control system and the intrathyroidal regulatory systems. TRH is produced by the superoptic and paraventricular nuclei within the hypothalamus and passes down their axones. Following secretion into the hypophyseal portal blood systems TRH passes to the pituitary and induces stimulation of TSH secretion.