Thyroid Function Test PDF

Thyroid function test Thyroid hormones regulate vital body functions: Breathing Heart rate Central and peripheral nervous systems Body weight Muscle strength Body temperature Cholesterol levels Thyroid stimulating hormones (TSH) is primarily respons...

Thyroid function test Thyroid hormones regulate vital body functions: Breathing Heart rate Central and peripheral nervous systems Body weight Muscle strength Body temperature Cholesterol levels Thyroid stimulating hormones (TSH) is primarily responsible for regulating thyroid function. TSH is produced by pituitary gland. The thyroid gland uses iodine to synthesize two main hormones: ❖ Tri-iodothyronine (T3) – biologically active form (10%) ❖ Thyroxine (T4) – inactive form, most abundant in circulation (90%) Tyrosine (amino acid) acts as a precursor for the synthesis of thyroid hormones. Thyroglobulin (protein) acts as the precursor for the thyroid hormone, and also represents a marker of thyroid cancer. Hence, it helps in storing thyroid hormones.  Iodine transport Na+ I (iodine) - symport is the transporter that is responsible for the transport of iodine into the thyroid cells against the concentration gradient.  Hypothyroidism Hashimoto’s disease is an autoimmune disorder in which the immune system creates antibodies that attack thyroid cells. Anti-thyroglobulin (anti-Tg) antibodies are specific for Hashimoto’s. Goiter is a condition where the thyroid gland grows larger. It occurs due to lack of iodine in diet.  Hypothyroidism may also be associated with an increased risk of heart disease and heart failure. Levothyroxine is considered as the gold standard for hypothyroidism treatment.  Hyperthyroidism Graves disease- diffuse goiter Anti-TSH receptor antibodies are specific for Grave’s disease. Radioactive iodine treatment is typically given for Grave’s hyperthyroidism. Radio Radioiodine is preferred as initial therapy for Grave’s disease in adults.  Hyperthyroidism Plummer disease- toxic multinodular goiter Thyroiditis- thyroid inflammation  Hyperthyroidism accelerates the body metabolism and causes unintentional weight loss, and a rapid or irregular heartbeat. Menopausal women are more prone to develop hyperthyroidism.  Carbimazole inhibits thyroid hormone synthesis, and also reduces the uptake of iodine into the thyroid cells. Its mode of action involves the inhibition of thyroid peroxidases (TPO) and inhibits organification of iodide and coupling of iodothyronines, thereby blocking thyroid hormone synthesis. Thyroid function test 1. Plasma protein–bound iodine This measure the amount of thyroid hormones bound to plasma proteins. Normally it is 4-8 mg per deciliter In hyperthyroidism it is above 8 mg per deciliter In hypothyroidism it is below 4 mg per deciliter 2. Measurement of the total thyroid hormone blood level It measures to protein-bound and free forms of T3 and T4 T4 = 5-12 µg/dL T3 = 80-220 ng/dL It is increased in hyperthyroidism. It is decreased in hypothyroidism. 3. Radioactive iodine uptake Small dose of I-131 is given orally or intravenously and the rate of its uptake by the thyroid gland is measured by gamma ray counter placed over the neck. The uptake is increased in hyperthyroidism and decreased in hypothyroidism. 4. Measurement of plasma thyroid stimulating hormone (TSH) Normal level is 0.4-4 milliunits/L Increased in primary hypothyroidism. It is decreased in hyperthyroidism and secondary hypothyroidism. It is the most accurate measure of thyroid activity. Analysis and interpretation  Medications that interfere with thyroid function testing ❖ Estrogens, such as in birth control pills, or in pregnancy, cause high levels of total T4 and T3. ❖ Biotin (vitamin B7), a commonly taken over-the-counter supplement, can cause the measurement of several thyroid function tests to appear abnormal, when they are in fact normal in the blood. Hence, biotin should not be taken for 2 days before blood is drawn for thyroid function testing to avoid this effect.