Thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), are essential for regulating various bodily functions[16][34]. The thyroid gland produces these hormones, which significantly influence metabolism, growth, and overall hormonal balance. The main job of thyroid hormones is to control the speed of your metabolism, which is how your body transforms food into energy. A well-functioning thyroid affects the body by maintaining body temperature, heart rate, and the rate at which you burn calories.

The thyroid gland, located in the anterior neck, produces several key hormones:

Thyroxine (T4): Also known as tetraiodothyronine, T4 is the primary hormone released by the thyroid. It contains four iodine atoms and serves as a precursor to T3[29]. The thyroid produces most of this hormone, but it has little effect on metabolism. Once released, organs in the body convert it to T3.
Triiodothyronine (T3): Contains three iodine atoms and is more potent than T4. It directly influences target tissues and broadly affects metabolic activity. T3 has a much greater effect on your metabolism than T4.
Reverse Triiodothyronine (RT3): The thyroid makes small amounts of RT3, which reverses the effects of T3.
Calcitonin: This hormone helps regulate the amount of calcium in the blood, but it doesn’t impact metabolism like T3 and T4.

The production and release of T3 and T4 are controlled by a feedback loop involving the hypothalamus, pituitary gland, and thyroid gland[9]. The hypothalamus releases thyrotropin-releasing hormone (TRH), which signals the pituitary gland to produce thyroid-stimulating hormone (TSH). TSH then triggers the thyroid to produce T4 and T3. Adequate iodine intake is essential for the thyroid to create T4 and T3. 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.

Once released, thyroid hormones affect nearly every cell and organ in the body[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[9][16]. Thyroid hormone (T3 and T4) affects every cell and all the organs in your body by:

Regulating the rate at which your body uses calories (energy). This affects weight loss or weight gain and is called the metabolic rate.
Slowing down or speeding up your heart rate.
Influencing the speed at which food moves through your digestive tract.
Controlling the way your muscles contract.
Managing skin and bone maintenance by controlling the rate at which your body replaces dying cells.

Thyroid hormones, particularly T3, increase the basal metabolic rate (BMR), which is the amount of energy the body uses while at rest[4][12][16]. They stimulate metabolic activity in almost all tissues of the body, increasing oxygen consumption and heat production[16]. T3 facilitates the synthesis of proteins, including enzymes, which boosts the basal metabolic rate. Thyroid hormones stimulate metabolic cycles involving fat, glucose, and protein catabolism and anabolism.

Thyroid hormones influence various physiological processes[16][34]:

Growth and Development: Thyroid hormones are essential for normal growth and development, especially in skeletal tissues. Adequate amounts of thyroid hormones are also needed for brain tissue development in children.
Cardiovascular System: Thyroid hormones help regulate blood pressure, heart rate, and the force and vigor of the heart's contraction.
Nervous System: Thyroid hormones are also vital for neurological function. Too much or too little thyroid hormone can cause mood changes, such as depression or anxiety.
Digestive System: Thyroid hormones affect how food moves through your digestive system.
Reproductive System: If your thyroid isn’t working properly, it can cause irregular menstrual periods and issues with fertility.

Abnormal thyroid hormone levels can result from several conditions[10][11][24]:

Hyperthyroidism: Occurs when the thyroid gland produces too much thyroid hormone, speeding up metabolism and potentially causing weight loss, rapid heart rate, and nervousness[24]. Graves’ disease is the most common cause[11].
Hypothyroidism: Occurs when the thyroid gland does not produce enough thyroid hormone, slowing down metabolism and potentially causing tiredness, weight gain, and depression[24]. Hashimoto’s thyroiditis is a common cause[11][24].
Goiter: An enlargement of the thyroid gland[11]. It can result from iodine deficiency, thyroid nodules, or autoimmune diseases[11][19].
Thyroid Nodules: These are lumps or swelling in the thyroid gland, which are often benign[19][24]. However, they can sometimes be cancerous or cause hyperthyroidism[19].
Thyroid Cancer: Cancer that forms in the tissues of the thyroid gland.

If you experience potential thyroid problem symptoms, a doctor can help you know if you have a thyroid problem. A doctor can order blood tests to check the levels of thyroid hormones in your body. 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. Treatment options include:

Medications: Thyroid hormone pills (synthetic thyroxine (T4)) can treat an underactive thyroid. Antithyroid medications block the thyroid's ability to make hormones.
Radioiodine therapy: A large dose of radioactive iodine damages your thyroid gland.
Surgery: Surgery can be performed to remove your thyroid gland.

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