TSH stands for thyroid-stimulating hormone. This hormone is produced by the pituitary gland, a small gland at the base of the brain, and starts a complex process that regulates endocrine function throughout the rest of the body. Because of its regulatory capacity, TSH will respond to any changes that occur to the body, especially if they are induced by exercise.
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TSH levels are a measure of the amount of thyroid stimulating hormone in your blood. TSH tells the thyroid gland -- which is situated in the neck -- to synthesize and release the hormones thyroxine (T4) and triiodothyronine (T3). Both of these hormones influence and control the body’s metabolism.
When they receive a signal from TSH, cells in the thyroid gland absorb iodine and combine it with the amino acid tyrosine -- a building block of protein -- to produce T3 and T4. A normal thyroid gland produces about 80 percent T4 and 20 percent T3. However, T3 possesses about four times the hormonal strength. Both hormones are then secreted into the blood, causing the levels of TSH to fall. In this way, the thyroid gland is like a furnace and the pituitary gland is like a thermostat. The levels of TSH and T3/T4 are always in balance.
Exercise and TSH Levels
According to a study published in the journal "Neuroendocrinology Letters" by researchers from the University of Gaziantep in Turkey, exercise performed at the anaerobic threshold -- meaning 70 percent of maximum heart rate and lactate produced in the muscles -- caused the most prominent increases in TSH, T3 and T4 levels. The rate of T4 and TSH continued to rise to 90 percent of maximum heart rate, but the rate of T3 began to fall beyond the anaerobic threshold.
Effects of Exercise
The relationship between exercise and the metabolism is intimately connected. When your activity levels increase, your body must also increase heart rate and the conversion of oxygen and calories to a molecule known as ATP, the main energy carrier of the cells. Increased thyroid activity is thought to be associated with a higher efficiency of the mechanical work performed by the exercising muscles. Other factors might play a role, including environmental temperature and the level of training, according to research published in the journal "Neuroendocrinology Letters." Evidence also suggests that if exercise-related energy expenditure exceeds caloric intake, then a low T3 state might be induced.
The level of TSH naturally follows the circadian rhythm, the roughly 24-hour cycle that governs biochemical, physiological and behavioral processes in living organisms. A study published in the "American Journal of Physiology" found that TSH increases to a maximal level at night. The timing of your workouts, however, can alter your TSH rhythm. Exercising at night can delay the TSH rhythm by 1 or 2 hours, according to a separate study published in the "American Journal of Physiology."