Chapter 5: Metabolism: Energy and Enzymes In this chapter you will
learn about the numerous chemical reactions in our bodies involved in breaking down food to produce essential
biological molecules and energy.
4 5.3 Metabolic Rate and the Thyroid and Parathyroid Glands
The thyroid gland is a large gland in the neck.
• It produces triiodothyronine
(T3) and thyroxine (T4) hormones,
using high levels of iodine acquired
from the bloodstream. Iodine in the
bloodstream comes from food that has
been consumed; iodized salt is the
• T3 and T4 increase metabolic rate by stimulating cells to break down more glucose and use
The thyroid also produces the hormone calcitonin. Calcitonin is released when blood
calcium levels rise. This promotes the deposit of calcium in bones.
. 5 Parathyroid Glands The parathyroid glands are embedded behind the thyroid gland.
• They produce parathyroid hormone (PTH)
• PTH causes blood levels of phosphate to decrease
• PTH also increases blood calcium levels by promoting release of calcium from bones,
reabsorption of calcium in the kidneys, and calcium absorption in the intestine
. 6 Regulation of Blood Calcium
Figure 5.8 McGraw-Hill Regulation of blood calcium level. Top: When the blood calcium
(Ca2+) level is high, the thyroid gland secretes calcitonin. Calcitonin promotes the uptake of
Ca2+ by the bones, and therefore the blood Ca2+ level returns to normal.
Bottom: When the blood Ca2+ level is low, the parathyroid glands release parathyroid hormone
(PTH). PTH causes the bones to release Ca2+ and the kidneys to reabsorb Ca2+ and activate
vitamin D. Thereafter, the intestines absorb Ca2+. Therefore, the blood Ca2+ level returns to