Thyroid function tests

The thyroid gland is located at the front of the neck just below the Adam’s apple (larynx).
It is butterfly-shaped and consists of two lobes located either side of the windpipe (trachea).
The thyroid gland is 2 inches (5 centimeters) wide and it weighs between 20 and 60 grams (0.7 to 2.1 ounces),

1. THYROID FUNCTION TESTS Dr.SHAISTA SUMAYYA, PHARM D SULTAN UL ULOOM COLLEGE OF PHARMACY, HYD GUIDED BY- Dr.S.P . SRINIVAS NAYAK , ASSISTANT PROFESSOR , SUCP , HYD

2. THYROID GLAND The thyroid gland is located at the front of the neck just below the Adam’s apple (larynx). It is butterfly-shaped and consists of two lobes located either side of the windpipe (trachea). The thyroid gland is 2 inches (5 centimeters) wide and it weighs between 20 and 60 grams (0.7 to 2.1 ounces),

3. THYROID HORMONES- Microscopically, the functional unit of the thyroid gland is the spherical thyroid follicle, lined with follicular cells (thyrocytes), and occasional parafollicular cells that surround a lumen containing colloid. The thyroid gland secretes three hormones: 2 thyroid hormones – triiodothyronine ( T3 ) and thyroxine ( T4 ) Calcitonin- a peptide hormone The thyroid hormones influence the metabolic rate and protein synthesis , and in children, growth and development. Calcitonin plays a role in calcium homeostasis . Secretion of the two thyroid hormones is regulated by thyroid-stimulating hormone (TSH), which is secreted from the anterior pituitary gland. TSH is regulated by thyrotropin-releasing hormone (TRH), which is produced by the hypothalamus.

4. FUNCTIONS OF THYROID HORMONES _- The thyroid gland produces hormones that regulate the body’s metabolic rate controlling heart, muscle and digestive function, brain development and bone maintenance. Its correct functioning depends on a good supply of iodine from the diet. Cells producing thyroid hormones are very specialized in extracting and absorbing iodine from the blood and incorporate it into the thyroid hormones

5. SYNTHESIS OF THYROID HORMONES- Thyroid hormone biosynthesis involves five steps. They are as follows: Iodide trapping Oxidation of iodide and iodination of tyrosine residues Hormone storage in the colloid of the thyroid gland as part of the large thyroglobulin molecule Proteolysis and release of hormones Conversion of less active prohormone thyroxine to more potent hormone 3,5,3-triiodothyronine. The first four steps are regulated by pituitary thyroid-stimulating hormone (TSH).

6. REGULATION MECHANISM- The production of thyroxine and triiodothyronine is primarily regulated by thyroid-stimulating hormone (TSH), released by the anterior pituitary gland . TSH release in turn is stimulated by thyrotropin releasing hormone (TRH), released in a pulsatile manner from the hypothalamus. The thyroid hormones provide negative feedback to the thyrotropes TSH and TRH: when the thyroid hormones are high, TSH production is suppressed. This negative feedback also occurs when levels of TSH are high, causing TRH production to be suppressed.
TRH is secreted at an increased rate in situations such as cold exposure in order to stimulate thermogenesis In addition to being suppressed by the presence of thyroid hormones, TSH production is blunted by dopamine, somatostatin, and glucocorticoids.

7. THYROID FUNCTION TESTS- Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy. It is also requested routinely in conditions linked to thyroid disease, such as atrial fibrillation and anxiety disorder. These tests include – Thyroid-stimulating hormone test (TSH), the thyroxine test (T4), the triiodothyronine test (T3), the thyroxine-binding globulin test (TBG), the triiodothyronine resin uptake test (T3RU), and the long-acting thyroid stimulator test (LATS), radioactive iodine uptake test , antithyroid antibodies.

8. TSH TEST – TSH “tells” the thyroid gland to secrete the hormones thyroxine (T4) and triiodothyronine (T3). Before TSH tests were used, standard blood tests measured levels of T4 and T3 to determine if the thyroid gland was working properly. Normal TSH levels for adults are - 0.5-5.0 mU/L T3 TEST- The triiodothyrine (T3) test measures the amount of this hormone in the blood. T3 is normally present in very small amounts, but has a significant impact on metabolism. It is the active component of thyroid hormones Normal T3 levels are : 90-170 ng/dl at birth
115-190 ng/dl at 6 to 12 years
110-230 ng/dl in adulthood

9. T4 TEST- Normal Total T4 levels are 10.1-2.0 ug/dl at birth
7.5-16.5 ug/dl at one to four months
5.5-14.5 ug/dl at four to 12 months
5.6-12.6 ug/dl at one to six years
4.9-11.7 ug/dl at 10 years
4-11 ug/dl at 10 years and older. NOTE : Levels of free T4 (thyroxine not attached to TBG) are higher in teenagers than in adults.

10. TBG- The thyroxine-binding globulin (TBG) test measures blood levels of this substance, which is manufactured in the liver. TBG binds to T3 and T4, prevents the kidneys from flushing the hormones from the blood, and releases them when and where they are needed to regulate body functions. NORMAL VALUES : 1.5-3.4 mg/dl or 15-34 mg/L in adults
2.9-5.4 mg/dl or 29-54 mg/L in children. RADIOIODINE UPTAKE STUDIES-

11. T3RU TEST- The triiodothyronine resin uptake (T3RU) test measures blood T3 levels. Laboratory analysis of this test takes several days, and it is used less often than tests whose results are available more quickly. Between 25% and 35% of T3 should bind to or be absorbed by the resin added to the blood sample . The test indirectly measures the amount of thyroid binding globulin (TBG) and thyroid-binding prealbumin (TBPA) in the blood. LATS TEST- The long-acting thyroid stimulator (LATS) test shows whether blood contains long-acting thyroid stimulator. Not normally present in blood, LATS causes the thyroid to produce and secrete abnormally high amounts of hormones. Long-acting thyroid stimulator is found in the blood of only 5% of healthy people.

12. ANTITHYROID ANTIBODIES- Antithyroid autoantibodies are autoantibodies targeted against one or more components on the thyroid. The most clinically relevant anti-thyroid autoantibodies are anti-thyroid peroxidase antibodies (anti-TPO antibodies, TPOAb), thyrotropin receptor antibodies (TRAb) etc.

13. THYROID DISORDERS- Disorders affecting thyroid gland are called as thyroid disorders Different types of thyroid disorders affect either its structure or function . There are specific kinds of thyroid disorders that includes:- Hypothyroidism Hyperthyroidism Goiter Thyroid nodules Thyroid cancer

14. HYPOTHYROIDISM- Hypothyroidism results from the thyroid gland producing an insufficient amount of thyroid hormone . It can develop from problems within the thyroid gland, pituitary gland, or hypothalamus. Some common causes of hypothyroidism include: Hashimoto’s thyroiditis (an autoimmune condition that causes inflammation of the thyroid gland) Thyroid hormone resistance Other types of thyroiditis (inflammation of the thyroid), such as acute thyroiditis and postpartum thyroiditis

15. HYPERTHYROIDISM- Describes excessive production of thyroid hormone , a less common condition than hypothyroidism. Some of the most common causes of hyperthyroidism are: Graves ’ disease Toxic multinodular goiter Thyroid nodules that overexpress thyroid hormone (known as “hot” nodules)

16. GOITER – A goiter simply describes enlargement of the thyroid gland, regardless of cause. A goiter is not a specific disease per se. A goiter may be associated with hypothyroidism, hyperthyroidism, or normal thyroid function. NODULES -Nodules are lumps or abnormal masses within the thyroid. Nodules can be caused by benign cysts, benign tumors, or, less commonly, by cancers of the thyroid. Nodules may be single or multiple and can vary in size. If nodules are excessively large, they may cause symptoms THYROID CANCER - Thyroid cancer Is far more common among adult women than men or youth. About 2/3 of cases occur in people under age 55. There are different kinds of thyroid cancer, depending upon the specific cell type within the thyroid that has become cancerous related to compression of nearby structures.