How Too Much Thyroid Hormone in the Blood Can Affect the Body

[00:00:01]The thyroid is the body's largest endocrine gland and is located anterior and lateral to the trachea and inferior to the larynx. One of the functions of the thyroid is to secrete thyroid hormone, which increases metabolic rate by increasing respiratory rate, increasing heart rate, accelerating breakdown of food for fuel, and increasing heat production.

[00:00:39]The thyroid hormone interacts with the hypothalamus and pituitary gland in a negative feedback loop to affect metabolic rate. Low levels of thyroid hormone in the blood cause the hypothalamus to release thyrotropin-releasing hormone.

[00:01:04]Thyrotropin-releasing hormone stimulates the anterior pituitary to release thyroid-stimulating hormone. Thyroid-stimulating hormone stimulates the thyroid gland to secrete thyroid hormones triiodothyronine, or T3, and thyroxine, or T4, into the bloodstream to affect cells throughout the body.

[00:01:38]The elevated levels of thyroxine in the blood are detected by the hypothalamus which stops production of thyrotropin-releasing hormone.

[00:01:53]This negative feedback loop keeps the metabolic rate from increasing too much.

[00:02:03]Hyperthyroidism is a condition in which there is a chronic high level of thyroid hormone present in the blood. The most common cause is Grave's disease, an idiopathic autoimmune disorder. In this condition, B cells in the thyroid mistakenly produce thyroid-stimulating immunoglobulins, which mimic the effect of thyroid-stimulating hormone and bind to the same receptor. As a result, binding these immunoglobulins stimulates the thyroid gland to continue producing thyroid hormone, which causes chronic manifestations of increased respiration, increased heart rate, called tachycardia, and increased heat production.

[00:03:06]In addition, the thyroid gland increases in size, resulting in a condition called thyroid hypertrophy or a goiter. Three common treatments of hyperthyroidism reduce circulating thyroid hormone. Antithyroid drugs such as propylthiouracil and methimazole suppress the synthesis of thyroid hormone. Surgical removal of thyroid tissue can reduce the amount of secreted thyroid hormone.

[00:03:48]Oral ingestion of radioactive iodine destroys thyroid tissue, resulting in decreased secretion of thyroid hormone.

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