Thyroid Function Explained - Part 1


Whithin the the endocrine system, the thyroid is the biological engine that ultimately directs hormonal function and, therefore, metabolism. The thyroid gland produces the hormones that make the body burn calories.

Thyroid hormones are what drive your basal metabolic rate ( the energy required for internal or cellular work when the body is at rest), in other words your metabolism.

The thyroid is a large, butterfly-shaped gland with two lobes connected by a body (or isthmus) over the trachea. Embedded within the thyroid gland are four masses of tissue called the parathyroid glands. The thyroid produces and secretes three major hormones: thyroxine, triiodothyronine, and calcitonin. Thyroid hormones influence almost every cell of the body. The thyroid glans has a crucial role in metabolism, fat burning, and oxygen utilization, as well as in gastrointestinal and neuromuscular function. That's why prolonged symptoms of heartburn or extended muscle aches are a sign of low thyroid function, Are we learning just how important the thyroid is yet?.

Thyroxine and triiodothyronine are produced when iodine combines with the amino acid tyrosine. Thyroxine (t4) is tyrosine bound to four molecules of iodine. Triiodothyronine (t3) is tyrosine bound to three molecules of iodine...understand how they came up with the term t4 and t3!. Iodine and tyrosine must be present in adequate amounts in the diet for the synthesis of t4. When thyroid hormone was first discovered, t4 was given exclusive credit for the metabolic activity at the cellular level, it was later discovered that t3 was four times more active than t4 at the target cells. It is now understood that much of the circulating t4 is actually converted to t3 prior to cellular metabolic activity.

The anterior pituitary gland and the hpta (hypothalamus) regulate thyroid hormone levels. Initially, the hpta responds to a metabolic change such as low body temp, stress, or sleep be releasing thyrotropin releasing factor (TRF), while simultaneously signaling the anterior pituitary gland to release thyroid stimulating hormone (TSH). TSH stimulates the thyroid, which traps iodine, synthesizes T4, and releases the thyroid hormone. See the importants of iodine in your diet yet?. As levels of t4 increase, the activities of the hpta and pituitary are inhibited (that didn't take long did it?).

T4 increases the metabolic rate of almost every tissue in the body. It's effects on metabolism are astonishing. For example, a person whose thyroid gland reduces the production of T4 will experience as much as a 40% drop in metabolism, or basal metabolic rate (the rate at which the bodyspends energy for the maintenance activities of the body). Meanwhile, overproduction of T4 can increase normal metabolic activity by 100%. T4 increases the basal metabolic rate (BMR) by impacting the rate of ATP (energy) production in the mitochondria ( the energy producing component of cells). The thyroid uses much of this energy to convert caloric energy to heat in a process called Thermogenesis. In other words, how your body uses food determines your metabloism.

Thyroid hormone increases the utilization of carbs and fat from food, and the rate of protien synthesis. It stimulates the appetite and the movement of food through the digestive you didn't know that?. In the presence of thyroid hormone, muscle catabolism increases, which increases the resting metabolic rate (muscle burns more energy than fat). Thyroid hormone also increases the sensitivity of skeletal muscle to impulses from the spinal cord. ( An excess of thyroid hormone is known to cause tremors, and a deficiency results in sluggish muscle response.) Thyroid hormone increases the uptake of oxygen into the cells, which speeds aerobic respiration. Finally, thyroid hormone actually increases the number of mitochondria whithin the cells.