How the Thyroid Works

Thyroid function involves a complex interplay of several organs, various hormones, and the right nutrients. In fact, thyroid function is directly affected by two other major organs — the hypothalamus and the pituitary gland. Together, the three organs form what is sometimes called the hypothalamic-pituitary-thyroid axis, or HPT axis. The way they operate provides a glimpse of your body's highly regulated system of checks and balances.

The hypothalamus is a region of the brain that acts as an internal regulation system. It controls certain metabolic processes and autonomic activities, such as breathing, swallowing, and blinking.

The hypothalamus also links the nervous system to the endocrine system through its production of neurohormones. Of particular importance to the thyroid gland is a neurohormone called thyrotropin-releasing hormone (TRH), also called TSH-releasing harmone. TRH levels are too low to be measured in the blood and so are never used to diagnose thyroid disease.

But when thyroid hormone levels are low, TRH stimulates the pituitary gland to produce thyroid-stimulating hormone (TSH). TSH, in turn, acts on the thyroid gland to produce thyroid hormone.

But the pituitary gland doesn't sit back after the thyroid hormone is released. It continues to monitor and assess the amount of hormone in the blood. If thyroid hormone drops too low, it releases more TSH to spur on greater production of thyroid hormone. If the amount of thyroid hormone goes too high, then the pituitary gland stops releasing TSH.

TSH levels can sometimes change even when your thyroid hormones are in the normal range. But usually, when the levels of thyroid hormone are just right, the pituitary maintains its production of TSH. That's why the measure of TSH is considered the most telling of your thyroid hormone levels.

Together, the hypothalamus and the pituitary and thyroid glands work together with help from other parts of the brain to ensure that your body cells work at the proper speed. In a healthy person, this well-orchestrated feedback loop keeps your body cells functioning the way they should, much in the same way that a thermostat ensures that your house stays at a stable temperature. Even the slightest increase or decrease, however, can alter the activity in your cells. That's when disease sets in.

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