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Thyroid hormones are essential to the proper development and differentiation of almost every cell of the human body. They affect metabolic rate and protein synthesis, regulate growth hormones and neural maturation, and increase the body's sensitivity to catecholamines (such as adrenaline). They regulate protein, fat, and carbohydrate metabolism, and stimulate vitamin metabolism.
Stiff-person syndrome patients may have co-existing autoimmune thyroid disease such as Graves’ disease and Hashimoto’s thyroiditis (described in the Co-existing Disease section).
Thyroglobulin is the protein precursor of the thyroid hormone made by thyroid cells, both healthy cells and cancerous cells. This test is primarily used as a tumor marker and to determine the presence of thyroid cells after thyroidectomy or radiation. If postoperative Tg levels are low, very little thyroid tissue remains.
TSH (thyroid stimulating hormone) is used to diagnose thyroid and pituitary disorders. It is used to screen for secondary hypothyroidism, Graves’ disease, Hashimotos thyroiditis, pituitary TSH-secreting tumors, and pituitary hypofunction.
T4 is the measurement of total thyroxine in circulation.
Free T4 is the measurement of T4 activity in the body.
T3 is the measurement of total triiodothyronine in circulation.
Free T3 is a measurement of T3 activity in the body.
Reverse T3 is a measurement of total reverse T3 in circulation and can indicate euthyroid sick syndrome resulting from decreased clearance while its production stays the same. The decreased clearance is possibly from lower 5'-deiodinase activity in the peripheral tissue or decreased liver uptake of rT3. It can also indicate a problem with T4 to T3 conversion.
Free thyroxine index is total thyroxine multiplied by thyroid hormone uptake, which, in turn, is a measure of the unbound thyroxine-binding globulins.
Thyroid levels are used to screen for hypothyroidism, Hashimoto’s thyroiditis, hyperthyroidism, Graves’ disease, congenital cretinism, iodine toxicity, pituitary tumors, and pituitary malfunction.
Triiodothyronine (T3)resin uptake test. measures the saturation of binding spots on thyronine-binding globulin (TBG) by endogenous T3. The test is performed by taking a blood sample, to which an excess of radioactive exogenous T3 is added, followed by a resin that also binds T3. A fraction of the radioactive T3 binds to sites on TBG not already occupied by endogenous thyroid hormone, and the remainder binds to the resin. The amount of labeled hormones bound to the resin is then be subtracted from the total that was added, with the remainder thus being the amount that was bound to the unoccupied binding sites on TBG.
Thyroxine-binding globulin (TBG) measures the the major thyroid protein carrier. When it is elevated, T3 and T4 are usually elevated. It is used to screen for hyperthyroidism, hypoproteinemia, pregnancy, estrogen-producing tumors, infectious heptatits, malnutrition, testosterone producing tumors, ovarian failure, major stress, and acute intermittment porphyria.
Thyroid antibody tests are used to screen for chronic thyroiditis, Graves’ disease, Hashimoto’s thyroiditis, and thyrotoxicosis.
TSI (thyroid stimulating immunoglobulin) measures antibodies directed against the thyroid cell receptor for TSH. This instigates stimulation of the thyroid gland independent of normal feed-back regulated TSH.
TBII (thyroid-binding inhibitory immunoglobulin) measures the ability of antibodies to inhibit TSH binding to its receptor, reflecting the presence of either or both stimulatory and inhibitory immunoglobulins.
Thyroid-stimulating hormone receptor (TSHR) antibodies are present in 70-100% of Graves' disease (85-100% for activating antibodies and 75-96% for blocking antibodies) and 1-2% of normal individuals.
Thyroglobulin antibodies are specific for thyroglobulin, a 660kDa matrix protein involved in the process of thyroid hormone production. They are found in 70% of Hashimoto's thyroiditis, 60% of idiopathic hypothyroidism, 30% of Graves' disease, a small proportion of thyroid carcinoma, and 3% of normal individuals.
The pituitary gland responds by sending a chemical message via thyroid stimulating hormone (TSH) to the thyroid gland.
In the thyroid gland, thyroglobulin and thyroperoxidase (an enzyme that frees iodine for addition onto tyrosine residues on thyroglobulin) produce the thyroid hormones: thyroxine (T4) (93%), triiodothyronine (T3 (6.1%) and a small amount of reverse T3 (0.9%). T0, T1, and T2 are hormone precursors and byproducts of thyroid hormone synthesis. They do not act on the thyroid hormone receptor and appear to be totally inert.
T4 and T3 are partially bound to thyroxine-binding globulin (TBG), transthyretin, and albumin. Only a very small fraction of the circulating hormone is free (unbound): 0.03% of T4 and 0.3% of T3. Only the free fraction has hormonal activity.
Thyroid hormone activity is regulated by a system of enzymes that activate, inactivate, or simply discard the prohormone T4 and in turn functionally modify T3 and rT3. These enzymes operate under complex direction of systems including neurotransmitters, hormones, markers of metabolism, and immunological signals. T4 is converted to T3 in target tissues: 40% in the liver, 40% in the gut, 20% in other tissues, and 20% becomes inactive reverse T3.
The hypothalamus receives the message that your cells need more T3. It sends a chemical message via thyroid releasing hormone (TRH) to the pituitary gland. TRH can also be detected in other areas of the body including the gastrointestinal system, pancreatic islets, and brain.
TPO (thyroid peroxidase antibodies) Anti-TPO antibodies are present in 99% of cases where thyroglobulin antibodies are present, however only 35% of anti-TPO antibody positive cases also demonstrate thyroglobulin antibodies.
Thyroid Scannning is done by having a patient swallow a radioactive iodine-131 capsule and 24 hours later undergoes an imaging scan. The test can also be performed with intravenous technetium, in which case the scan can be performed 2 hours after injection. It is used in detecting thyroid abnormalities such as goiters and tumors, adenomas, cysts, lymphoma, hyperthyroidism, and hypothyroidism. It can differentiate Graves’ disease from Plummer disease. It can exclude the thyroid as a primary site in metastatic cancers.
PTH (PTH), parathormone or parathyrin, is secreted by the parathyroid glands and increases the concentration of calcium in the blood by acting upon the parathyroid hormone 1 receptor (high levels in bone and kidney) and the parathyroid hormone 2 receptor (high levels in the central nervous system, pancreas, testis, and placenta.
Calcitonin is a hormone produced by the thyroid gland that acts to decrease calcium concentration.
For more information on these tests, visit Quest Labs.
T3 is transported into the (TRH)-producing neurons in the paraventricular nucleus. These neurons express MCT8, a thyroid hormone transporter. T3 then binds to the thyroid hormone receptor in these neurons and affects the production of thyrotropin-releasing hormone, thereby regulating thyroid hormone production.
T3 is absorbed into cells via receptors. It increases the rate of protein synthesis and protein degradation, potentiates the effects of the β-adrenergic receptors on the metabolism of glucose, stimulates the breakdown of cholesterol and increases the number of LDL receptors, increases the heart rate and force of contraction, increasing systolic and decreasing diastolic blood pressure. T3 may increase serotonin in the cerebral cortex, and down-regulate 5HT-2 receptors.