Thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), are essential for regulating various bodily functions[5][16][34]. The thyroid gland produces these hormones, which significantly influence metabolism, growth, and overall hormonal balance[5]. The main job of thyroid hormones is to control the speed of your metabolism, which is how your body transforms food into energy[2][6]. A well-functioning thyroid affects the body by maintaining body temperature, heart rate, and the rate at which you burn calories[1].
The thyroid gland, located in the anterior neck, produces several key hormones[5]:
The production and release of T3 and T4 are controlled by a feedback loop involving the hypothalamus, pituitary gland, and thyroid gland[2][5][9]. The hypothalamus releases thyrotropin-releasing hormone (TRH), which signals the pituitary gland to produce thyroid-stimulating hormone (TSH)[2][5]. TSH then triggers the thyroid to produce T4 and T3[2]. Adequate iodine intake is essential for the thyroid to create T4 and T3[2][5]. When T3 and T4 levels increase, they prevent the release of TRH (and thus TSH), and when T3 and T4 levels drop, the feedback loop starts again[2].
Once released, thyroid hormones affect nearly every cell and organ in the body[2][4][9][14][16]. Thyroxine (T4) enters cells, it is converted to triiodothyronine (T3), which binds to thyroid receptors in the nucleus to regulate the transcription of specific genes[3][9][16]. Thyroid hormone (T3 and T4) affects every cell and all the organs in your body by:
Thyroid hormones, particularly T3, increase the basal metabolic rate (BMR), which is the amount of energy the body uses while at rest[4][5][12][16]. They stimulate metabolic activity in almost all tissues of the body, increasing oxygen consumption and heat production[5][16]. T3 facilitates the synthesis of proteins, including enzymes, which boosts the basal metabolic rate[5]. Thyroid hormones stimulate metabolic cycles involving fat, glucose, and protein catabolism and anabolism[3].
Thyroid hormones influence various physiological processes[4][5][16][34]:
Abnormal thyroid hormone levels can result from several conditions[2][6][10][11][24]:
If you experience potential thyroid problem symptoms, a doctor can help you know if you have a thyroid problem[1][6]. A doctor can order blood tests to check the levels of thyroid hormones in your body[1][6]. A doctor might also give you radioactive iodine by mouth or as an injection to measure how much of it your thyroid gland takes up, as taking in a lot of radioactive iodine is a sign that your thyroid is overactive[1]. Treatment options include:
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