Thyroid function tests for MBBS, LAB. MED & BDS.pptx

Thyroid function tests (TFTs) are the most frequently ordered endocrine investigations in children and adolescents.
Abnormalities in TFTs can help in diagnosis of primary thyroid disorders (i.e. disorders in which the defect is at the thyroid level) as well as secondary or central thyroid disorders (in which defect is at the pituitary level).

1. Thyroid Function Tests Rajendra Dev Bhatt, PhD Scholar Asst. Professor/Clinical Biochemist Clinical Biochemistry & Laboratory Medicine Fellow: Translational Research (2018-2022) in CVD in Nepal, NHLBI & NIH, USA

2. Hypothyroidism is the most common thyroid disorder in the pediatric (as well as adult) population . In recent years, there appears to be an increase in the use of TFTs by clinicians during routine health assessments.

3. Introduction Thyroid function tests (TFTs) are the most frequently ordered endocrine investigations in children and adolescents. Abnormalities in TFTs can help in diagnosis of primary thyroid disorders (i.e. disorders in which the defect is at the thyroid level) as well as secondary or central thyroid disorders (in which defect is at the pituitary level).

4. T hyroid Gland The thyroid gland, located immediately below the larynx on each side of and anterior to trachea, is one of the largest of endocrine glands. It secretes two major hormones Thyroxine and triidothyronine . It also secretes calcitonin, an important hormone for calcium meatabolism

5. Thyroid Hormones Thyroid hormone are potent regulator of cellular proliferation and metabolic rate and must be maintained within an optimal range for normal development and health . Thyroid hormone metabolism , which describes the biochemical activation and inactivation of thyroid hormones, is a powerful mechanism regulating thyroid hormone action.

6. Thyroid Hormone Synthesis and Plasma Transport Thyroid hormone secretion is regulated by the hypothalamic-pituitary-thyroid axis. Hypothalamus produces thyrotropin releasing hormone (TRH), which stimulates the pituitary to secrete thyrotropin . Thyrotropin , also called thyroid stimulating hormone (TSH ), stimulates thyroid hormone synthesis and glandular secretion.

7. TRH Release (Hypothalamus )

8. TSH Release (Pituitary )

9. Thyroid Hormone Synthesis (Thyroid Gland)

10. Thyroglobulin Production The thyroid follicular cells produce a protein called thyroglobulin and release it into the colloid, a process known as exocytosis. The colloid is rich in thyroglobulin as a result. The TSH that is released from the anterior pituitary gland will travel through the bloodstream and bind to TSH receptors on thyroid follicular cells. When TSH binds to follicular cells, the production of thyroglobulin is enhanced.

11. The production of the thyroid hormone is initiated by the absorption of iodine from the GI tract, where the iodine is reduced to iodide and released into the plasma. Thyroid follicular cells are uniquely adapted to concentrate iodine and to incorporate it into thyroid hormones.

12. Iodide Uptake We mainly get iodine from our diet, which then gets absorbed from the intestines into the bloodstream as iodide.  In addition to enhancing thyroglobulin production, TSH also stimulates the follicular cells to increase iodide uptake from the blood.  The concentration of iodide is greater in the follicular cells than the blood, so iodide uses the help of sodium to travel against its concentration gradient. The phenomenon is known as iodide trapping . Sodium and iodide enter the follicular cells via a sodium/iodide symporter . TSH will increase the expression and activity of sodium/iodide symporters in order to facilitate increased iodide uptake. Once in the follicular cell, iodide will then enter the colloid via a transporter called pendrin .  Iodide is now in the colloid with thyroglobulin .

13. Iodide Oxidation The thyroid uses an enzyme called thyroid peroxidase (TPO) to oxidize iodide into iodine. This process is known as iodide oxidation because we are losing electrons to form iodine. TSH also increases the activity of TPO. Now we have our 2 ingredients to make thyroid hormone: Thyroglobulin and Iodine. 

14. Thyroglobulin Iodination Thyroglobulin contains tyrosine amino acids residue. The thyroid will again use the enzyme TPO to place iodine onto tyrosine, a process known as organification or iodination of thyroglobulin.  If one iodine is placed on tyrosine, then it becomes monoiodotyrosine (MIT). If two iodine are placed on tyrosine, then it becomes diiodotyrosine (DIT). MITs and DIT molecules then get combined to form T3 and T4, which are the main thyroid hormones. 

15. Thyroglobulin Proteolysis We now have thyroglobulin containing T3 and T4. Therefore, we have to cut the T3 and T4 out of the protein. In order to do this, thyroglobulin exits the colloid and enters back into the follicular cell, a process known as endocytosis. TSH increases thyroglobulin endocytosis. Lysosomes in the follicular cell fuse with the thyroglobulin endosome. The lysosomes contain proteolytic enzymes called proteases that cleave (split) the T3 and T4 from thyroglobulin.

16. Thyroid Hormone Release T3 and T4 will then exit the follicular cell and enter the bloodstream to act on target tissues and organs. The majority of T3 and T4 are bound to carrier proteins as they travel through the blood. One of the main carrier proteins is thyroxine-binding globulin (TBG) Although T3 and T4 can both have effects on target tissues, T3 is considered the active form of thyroid hormone. Much of T4 gets converted into T3 peripherally, with some of it getting converted in the thyroid.

17. Both active thyroid hormones , thyroxine (T4) and tri- iodothyronine (T3), exert negative feedback upon the hypothalamus and the pituitary. Assays to measure serum TSH, T4, and T3 are all routinely available in clinical laboratories and are adequate to diagnosis primary and central thyroid dysfunction.

18. Thyroid hormones are composed mostly of iodine (65% of T4’s weight; 58% of T3’s weight) which is primarily derived from the diet. In non-pregnant adults, a daily dietary iodine intake of 100–150 μg is sufficient to meet the synthetic requirements of the normal thyroid hormones.

19. The sodium/iodide symporter (NIS ) is a membrane protein on the basolateral surface of thyroid follicular cells that actively transports circulating iodide into the thyrocyte . Intracellular iodide is then oxidized and conjugated onto tyrosine residues of the large glycoprotein thyroglobulin via a process called “ organification ”.

20. This tyrosine iodination reaction can add either one iodine atom (resulting in monoiodotyrosine or MIT) or two (resulting in diiodotyrosine or DIT), and the subsequent “coupling ” of two iodinated tyrosines produces both T4 (formed by the coupling of two DIT molecules ) and T3 (formed by the coupling of one DIT molecule and one MIT molecule ).

21. Mature thyroglobulin thus contains both T4 and T3, present in a ratio of about 15:1 Thyroglobulin is stored within the center of thyroid follicles in a matrix called colloid and this constitutes a large reservoir of preformed hormone, equivalent to several weeks of normal secretion . This storage property is unique to the endocrine glands and permits continued thyroid hormone availability during transient iodine deficiency and other environmental goitrogen exposures which might otherwise temporarily impair thyroid hormone synthesis.

22. Thyroid hormone secretion begins with the endocytosis of colloid from the apical membrane of thyroid follicular cells . Thyroglobulin then undergoes lysosomal proteolysis to release T4 and T3 , which subsequently enter the circulation at a ratio of about 11 :1

23. Many of the biochemical reactions required for thyroid hormone synthesis are catalyzed by the thyroperoxidase enzyme. In addition, several other thyroid-specific proteins such as NIS, the thyroid oxidases which generate H2O2 for iodine oxidation, and the iodotyrosine deiodinases responsible for iodine recycling are also required for normal thyroid hormone synthesis.

24. Thyroid hormones have poor aqueous solubility and most T4 and T3 in the circulation are bound to plasma proteins. The major binding protein in humans is thyroxine-binding globulin, which binds about 68% of circulating T4 and 80% of circulating T3, followed by albumin and transthyretin (previously termed thyroxine binding prealbumin ).

25. Only about 0.02% of serum T4 and 0.3% of serum T3 is normally free (unbound) and available to enter cells and signal thyroid hormone action. An understanding of this “free hormone concept” is important in clinical practice , as several inherited and acquired conditions can alter the amount of these serum binding proteins and/or their affinity for thyroid hormones.

26. While free thyroid hormone concentrations are normal in such individuals, these binding abnormalities can mimic the laboratory findings of central thyroid disease and care must be taken to avoid misdiagnosis. For example, congenital thyroxine-binding globulin deficiency presents with a low serum total T4 and normal serum TSH ( mimicking central hypothyroidism ).

27. Serum thyroid hormone binding proteins serve two important physiologic functions . The first is to prolong the serum half-life of thyroid hormones by reducing their renal clearance. The second is to promote a homogenous level of T4 and T3 in the circulation.

28. Regulation of Thyroid Hormone Synthesis

29. Catabolism of Thyroid Hormones T4 has a half-life of 4-7 days, while T3 has about 1 day. T3 is biologically more active. T4 is a prohormone which is deiodinated to T3. In the peripheral tissues, de-iodination takes place . This is done by a deiodinase , a selenium containing enzyme .

30. Thyroid Hormone Action The hormone attaches to specific nuclear receptors. Then the receptor-hormone complex binds to the DNA . The T3 receptor complex binding sequence in the DNA or the thyroid responsive element (TRE ) has been identified The T3 binding results in increase in transcription rate.

31. Thyroid hormones enter cells through membrane transporter proteins. Once inside the nucleus, the hormone binds its receptor, and the hormone-receptor complex interacts with specific sequences of DNA in the promoters of responsive genes.  The effect of the hormone-receptor complex binding to DNA is to modulate gene expression , either by stimulating or inhibiting transcription of specific genes.

32. For the purpose of illustration, consider one mechanism by which thyroid hormones increase the strength of contraction of the heart. Cardiac contractility depends, in part, on the relative ratio of different types of myosin proteins in cardiac muscle. Transcription of some myosin genes is stimulated by thyroid hormones, while transcription of others in inhibited. The net effect is to alter the ratio toward increased contractility.

33. Metabolic Effects of Thyroid Hormones Thyroid hormones have profound effects on many physiologic processes, such as development , growth and metabolism. They stimulate diverse metabolic activities most tissues, leading to an increase in basal metabolic rate . One consequence of this activity is to increase body heat production , which seems to result, at least in part , from increased oxygen consumption and rates of ATP hydrolysis .

34. Lipid M etabolism: Increased thyroid hormone levels stimulate fat mobilization , leading to increased concentrations of fatty acids in plasma. They also enhance oxidation of fatty acids in many tissues. Finally, plasma concentrations of cholesterol and triglycerides are inversely correlated with thyroid hormone levels. One diagnostic indication of hypothyroidism is increased blood cholesterol concentration.

35. Carbohydrate Metabolism: Thyroid hormones stimulate almost all aspects of carbohydrate metabolism, including enhancement of insulin-dependent entry of glucose into cells and increased gluconeogenesis and glycogenolysis to generate free glucose.

36. Protein Metabolism: Earliest effect of T4 is stimulation of RNA synthesis and consequent increase in protein synthesis . Higher concentration of T3 causes protein catabolism and negative nitrogen balance .

37. Other Metabolic Effects On muscle: T3 increases glucose uptake by muscle cells it also stimulate protein synthesis and therefore growth of muscle through its stimulatory actions on gene expression. Thyroid hormone sensitizes the muscle cell to the glycogenolytic actions of epinephrine. Glycolysis in muscle is increased by this action of T3.

38. On the pancreas: Thyroid hormone increases the sensitivity of the  cells of the pancreas to those stimuli that normally promote insulin release and is required for optimal insulin secretion. On Cardiovascular system: Thyroid hormones increases heart rate, cardiac contractility and cardiac output. They also promote vasodilation, which leads to enhanced blood flow to many organs.

39. On Central nervous system: Both decreased and increased concentrations of thyroid hormones lead to alterations in mental state. Too little thyroid hormone, and the individual tends to feel mentally sluggish, while too much induces anxiety and nervousness . On Reproductive system: Normal reproductive behavior and physiology is dependent on having essentially normal levels of thyroid hormone. Hypothyroidism in particular is commonly associated with infertility.

40. Hypothyroidism Primary hypothyroidism: (Most common): --‐ Failure of thyroid gland . Secondary hypothyroidism: Failure of the pituitary to secrete TSH ( rare). Failure of the hypothalamic-pituitary‐thyroid axis .

41. Causes : Hashimoto’s disease. Radioiodine or surgical treatment of hyperthyroidism. Drug effects . TSH deficiency . Congenital defects . Severe iodine deficiency .

42. Hyperthyroidism Increased secretion of thyroid hormones. Tissues are exposed to high levels of thyroid hormones (thyrotoxicosis). Increased pituitary stimulation of the thyroid gland (secondary).

43. Causes : Grave’s disease. Toxic multinodular goiter Thyroid adenoma. Thyroiditis. High Intake of iodine / iodine drugs Intake of exogenous T4 and T3 .

44. TEST YOURSELF 1. The thyroid gland mostly secretes : A. T3 B. T4 C. Equal quantities D. N either B. T4

45. 2. T4 is mostly converted to T3 in Thyroid B. When Needed C. Peripheral Tissues D. Never converted C. Peripheral Tissues

46. 3. A patient is presented with diarrhea, palpitation and weight loss, he most probably has: Hyperthyroidism Hypothyroidism C. Euthyroid Hyperthyroidism

47. THANK YOU