The thyroid gland communicates with the hypothalamus, a part of the brain. The hypothalamus sends messages to the pituitary, a pea-sized gland connected to the hypothalamus. After receiving the messages, this gland, called the pituitary, produces thyroid stimulating hormone, or TSH, which acts on the thyroid gland to produce thyroid hormones. Thyroid hormone determines the metabolic rate of the body and affects nearly all cells. Production of thyroid hormone is closely linked to the amount of TSH circulating in the bloodstream. The communication between the thyroid gland and brain is known as the hypothalamic-pituitary-thyroid axis.
The hypothalamus is located in the lower central part of the brain. The nerve cells in the hypothalamus produce chemicals that stimulate the pituitary gland, also called the "master gland," because it controls the thyroid and other glands in the body. When the thyroid hormone concentration in the blood falls, the hypothalamus responds by making thyrotropin-releasing hormone, TRH, which stimulates the pituitary gland to make thyroid-stimulating hormone, TSH.
The pituitary gland, located at the bottom of the hypothalamus, consists of two distinct parts, the anterior and posterior lobes. The anterior pituitary lobe makes TSH. TSH stimulates the thyroid gland to make thyroid hormones. The amount of TSH produced is not only controlled by TRH from the hypothalamus, but also by the amounts of thyroid hormones, T4 and T3, in the bloodstream. If thyroid hormone T4 and T3 concentrations are high, the pituitary gland stops making TSH. This is called "negative feedback regulation." The pituitary gland is positively stimulated by the hypothalamus via TRH, and it is negatively regulated by T4 and T3 thyroid hormones.
The two lobes of the butterfly-shaped thyroid gland sit in front of the windpipe just below the Adam's apple. The thyroid gland produces mostly the thyroid hormone thyroxine, also called T4. For full activity, T4 has to be converted to T3. Body organs, such as the kidneys and liver and to some extend the thyroid gland itself, convert T4 into active T3. To make thyroid hormone, the thyroid gland needs iodine, which it obtains from the diet. Examples of sources of iodine are seafood and iodized salt. Thyroid disorders develop when insufficient amounts of iodine are available.
In healthy people, the hormones that communicate between the thyroid, pituitary and hypothalamus are balanced. This balance can disappear due to the lack of iodine, damage to the thyroid or pituitary, certain medications and autoimmune disorders. A breakdown in communication can result in the thyroid gland producing too much or too little thyroid hormone. The condition in which the thyroid pumps out too much hormone is called hyperthyroidism. Hyperthyroidism causes the heart rate to race and may cause you to feel overheated and irritable. Hypothyroidism is the opposite condition, in which the thyroid makes too little thyroid hormone, even when the pituitary gland produces lots of TSH. Low levels of thyroid hormones or hypothyroidism cause you to have low energy, make you feel cold, and you may gain weight even if you eat less.
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