Review

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The follicular cells of the thyroid gland secrete the thyroid hormones, thyroxine, also known as tetraiodothyronine (T₄), and triiodothyronine (T₃). Thyroxine (T₄) is converted to triiodothyronine (T₃) in the tissues. Thyroid hormones exert their influence in the body in a variety of ways. Three major effects are:

1. Increase the basal metabolic rate (BMR) by increasing use of oxygen to make ATP. Increasing BMR increases cellular metabolism of carbohydrates, lipids, and proteins.
2. Enhance the action of norepinephrine and epinephrine which results in increased heart rate, more forceful heart contractions, and increased blood pressure.
3. In conjunction with growth hormone and insulin, thyroid hormones accelerate body growth. They are required for normal growth and development of the nervous and skeletal systems.

Secretion of thyroid hormones is controlled by a negative feedback system. Thyrotropin-releasing hormone (TRH), secreted by the hypothalamus, stimulates the anterior pituitary to secrete TSH (thyroid stimulation hormone). TSH, in turn, stimulates the secretion of triiodothyronine (T₃) and thyroxine (T₄) by the thyroid gland. Increasing levels of triiodothyronine (T₃) and thyroxine (T₄) inhibit release of TRH and TSH.

Hypothyroidism occurs in three forms, and is described as low blood levels of thyroid hormones. Primary hypothyroidism is a decrease in secretion of thyroid hormones by the thyroid gland, most commonly due to a dietary deficiency of iodine or as a result of an autoimmune disorder. Secondary hypothyroidism occurs when the pituitary produces insufficient TSH. Tertiary hypothyroidism generally occurs when hypothalamic tumors prevent the secretion of normal amounts of TRH which also result in reduced secretion of thyroid hormones.

Hyperthyroidism is characterized by high blood levels of thyroid hormones due to increased secretion of thyroid hormones by the thyroid gland. The most common cause of hyperthyroidism is Graves' disease, an autoimmune disorder where an antibody is produced that mimics TSH.

Measurement of TSH, T₄, and Thyroid Activity

In this experiment, blood serum levels of T₄ and TSH will be measured. While T₃ is the most active form of the thyroid hormones, T₄ is the most stable form. T₄ also converts to T₃ in body cells thus serving as a source for T₃, so measurement of T₄ concentration provides better information on the thyroid status of a patient.

Blood samples are collected in a non-heparinized blood collecting tube and sent to a clinical laboratory for measurement of T₄ and TSH. Without heparin, blood clots, allowing the blood clot and serum to be separated by centrifugation. Serum is collected and used to measure T₄ by radioimmunoassay (RIA) and TSH by immunoradiometric assay (IRMA), IRMA is more sensitive than an RIA and is needed to measure the small changes in TSH that occur with hypothyroidism. The amount of iodine accumulated by the thyroid gland is an indication of thyroid activity. Thyroid imaging is used to observe the relative amount of iodine accumulated by the thyroid gland.