The Functioning of Thyroid Hormones

The thyroid gland, located in the anterior neck, produces several key hormones^[5]:

• Thyroxine (T4): Also known as tetraiodothyronine, T4 is the primary hormone released by the thyroid^[2][5]. It contains four iodine atoms and serves as a precursor to T3^[5][6][29]. The thyroid produces most of this hormone, but it has little effect on metabolism^[6]. Once released, organs in the body convert it to T3^[2].
• Triiodothyronine (T3): Contains three iodine atoms and is more potent than T4^[5][6]. It directly influences target tissues and broadly affects metabolic activity^[5]. T3 has a much greater effect on your metabolism than T4^[6].
• Reverse Triiodothyronine (RT3): The thyroid makes small amounts of RT3, which reverses the effects of T3^[6].
• Calcitonin: This hormone helps regulate the amount of calcium in the blood, but it doesn’t impact metabolism like T3 and T4^[2].

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:

• Regulating the rate at which your body uses calories (energy). This affects weight loss or weight gain and is called the metabolic rate^[2].
• Slowing down or speeding up your heart rate^[2].
• Influencing the speed at which food moves through your digestive tract^[2].
• Controlling the way your muscles contract^[2].
• Managing skin and bone maintenance by controlling the rate at which your body replaces dying cells^[2].

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]:

• Growth and Development: Thyroid hormones are essential for normal growth and development, especially in skeletal tissues^[5]. Adequate amounts of thyroid hormones are also needed for brain tissue development in children^[5].
• Cardiovascular System: Thyroid hormones help regulate blood pressure, heart rate, and the force and vigor of the heart's contraction^[4].
• Nervous System: Thyroid hormones are also vital for neurological function^[4]. Too much or too little thyroid hormone can cause mood changes, such as depression or anxiety^[1].
• Digestive System: Thyroid hormones affect how food moves through your digestive system^[4].
• Reproductive System: If your thyroid isn’t working properly, it can cause irregular menstrual periods and issues with fertility^[4].

Abnormal thyroid hormone levels can result from several conditions^[2][6][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^[1][4][24]. Graves’ disease is the most common cause^[1][4][11].
• Hypothyroidism: Occurs when the thyroid gland does not produce enough thyroid hormone, slowing down metabolism and potentially causing tiredness, weight gain, and depression^[1][4][24]. Hashimoto’s thyroiditis is a common cause^[1][4][11][24].
• Goiter: An enlargement of the thyroid gland^[4][11]. It can result from iodine deficiency, thyroid nodules, or autoimmune diseases^[4][5][11][19].
• Thyroid Nodules: These are lumps or swelling in the thyroid gland, which are often benign^[1][4][19][24]. However, they can sometimes be cancerous or cause hyperthyroidism^[1][4][19].
• Thyroid Cancer: Cancer that forms in the tissues of the thyroid gland^[1][4].

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:

• Medications: Thyroid hormone pills (synthetic thyroxine (T4)) can treat an underactive thyroid^[1][4]. Antithyroid medications block the thyroid's ability to make hormones^[6].
• Radioiodine therapy: A large dose of radioactive iodine damages your thyroid gland^[1].
• Surgery: Surgery can be performed to remove your thyroid gland^[1].