The thyroid gland consists of two lobes on either side of the larynx and upper trachea (Fig. 16-2). It secretes a mixture of hormones, mainly thyroxine (T4) and triiodothyronine (T3). Because thyroid hormones
contain iodine, their levels can be measured and the activity of the thyroid gland can be studied by following the uptake of iodine. Most thyroid hormone in the blood is bound to protein, mainly thyroid binding globulin (TBG).
On the posterior surface of the thyroid are four to six tiny parathyroid glands that affect calcium metabolism (Fig. 16-3). Parathyroid hormone increases the blood level of calcium. It works with the thyroid hormone thyrocalcitonin, which lowers blood calcium, to regulate calcium balance.
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