functions and metabolism of Thyroid gland and thyroid function tests

thyroid hormone synthesis, functions and metabolism

1. Functions and Metabolism of Thyroid Gland and Thyroid Function Tests By- Dr Surbhi Tyagi Moderator-Dr Ruchi

2. Learning objectives At the end we will be able to understand: Anatomy and physiology of thyroid gland Thyroid hormone synthesis, storage and metabolism Regulation of thyroid hormones Function of thyroid hormones and its clinical implication Tests to assess the thyroid gland function

3. ANATOMY Largest endocrine gland, consists of two lateral lobes connected by isthmus

4. Functional Units: Acini or follicles – responsible for synthesis and secretion of T3 and T4 20-40 follicles form a lobule Acini lining – single layer of cuboidal epithelium Parafollicular cells/ C Cells : produce and secrete calcitonin Thyroglobulin: Glycoprotein, synthesized by thyroid cells and secreted into colloid HISTOLOGY

5. Steps of synthesis, storage & release of thyroid hormones

6. Iodine metabolism Iodide uptake- Critical 1 st step in T₄ & T₃ synthesis Primary source- Diet Recommended dietary intake Adults: 150 - 250 µg/day Pregnancy: 250 µg/day Children: 90-120 µg/day Normal plasma Iodide level- 0.15-0.3 µg/dl Dietary Iodine ↓ Iodide(I⁻) in GIT ↓ Transport of plasma I⁻ into thyroid follicles ↓ Per oxidase Oxidized Iodine

7. Iodide trapping mechanism (Iodide pump) Iodide pump or Na/I Symporter Basolateral membrane of follicles Stimulated – TSH Inhibited – chlorate, perchlorate, thiocyanate, pertechnate , periodate Produces intracellular I⁻ conc. 20-40 times > than that of plasma

8. Oxidation & iodination :- Oxidized Iodine ( I, HOI, E-OI) bind to Tyrosine residues of TG to form MIT & DIT Coupling - MIT + DIT ----------------------> T₃ Per oxidase DIT + DIT -----------------------> T₄ MIT+ DIT -----------------------> RT₃ (inactive) Only 25% of MIT & DIT undergo coupling

9. Storage and Release – T3 and T4 remain attached to thyroglobulin and are stored as colloid Endocytosis ↓ Lysosomal protease Release Thyroid gland: MIT- 23% , DIT- 33% , T₄- 35% & T₃- 7% Peripheral conversion of T4 to T3– Occurs in liver, kidney and spleen All target tissues take up T3 from circulation, except brain & pituitary which convert T4 to T3 within own cells

10. PTU/ MMZ Iodide PTU/ MMZ Iodide PTU, Propranolol, corticosteroid blocker, Ipodate Thyroid Peroxidase

11. Structure of Thyroid Hormones

12. THYROID HORMONES Thyroxine(T4) - about 90% of the hormone secretion. - Up to 80% of the T4 is converted to T3 by D1 type Deiodinase in peripheral organs such as the liver, kidney and spleen. - half life: 6 – 7 days Triiodothyronine (3.5,3’,-triiodothyronine ) - about 9% of the hormone secretion - 10 times more active than thyroxine - T₃ is generated within target cells (skeletal muscles, heart, brain, pituitary) by D2 type Deiodinase - half life: 1 day

13. T₃ is the active hormone, while T₄ is mainly a transport form; functions as a prohormone of T₃ Reverse T3 – - 1% not biologically active Calcitonin - important role in calcium metabolism - antagonise PTH hormone

14. Transport Daily secretion: T₄ - 80µg, T₃ - 4µg & RT₃ -2µg Thyroid binding proteins: Thyroxine binding globulin (TBG) Thyroxine binding pre-albumin (TBPA) Albumin T 4 - predominantly bound to TBG 99.98% of T₄ → Bound (TBG) Total T₄ → 5-11 µg/dl, Free T₄ → 0.7-2.5 ng/dl (0.02%) T3 - predominantly bound to albumin 99.7% of T₃ is bound → 46% to TBG and rest to albumin Total T₃ → 70-190 ng /dl, Free T₃ → 0.2-0.5 ng /dl (0.3%) Physiologically active hormones → Free T₄ , T3

15. Alteration in binding of T4 &T3 to TBG

16. Metabolism T₄ & T₃ deiodinated in liver, kidney & other tissues by deiodinase . 3 different Deiodinase (D1, D2 & D3)

17. Regulation of Thyroid Secretion Regulated primarily by pituitary TSH TRH and T₄ ,T₃ act competitively on thyrotrophs Free T₄ ,T₃ ↓ TSH T₃ → main hormone for – ve feedback Other factors regulationg TSH : Oestrogen ↑ TSH Large doses of I⁻ ↓ T₄ & T₃ ↑ TSH Somatostatin ↓ TSH Dopa and Dopamine ↓ TSH Glucocorticoids ↓ TSH

18. Mechanism of action T₃ & T₄ penetrate cells by active transport & act by combining with nuclear thyroid hormone receptor (TR) TR ( α & β ) – steroid and retinoid family receptors TR residues bound to Thyroid hormone response element (TRE) in the enhancer region of target genes along with corepressors → gene transcription suppressed When T₃ binds to TR residues → RXR → conformational change releasing corepressor and binding coactivator ↓ Induces gene transcription → protein synthesis →metabolic effects

19. Functions of Thyroid gland Calorigenic (Heat production) action : T₄ & T₃ ↑ O₂ consumption and activity of Na-K pump of all metabolically active tissues → ↑Heat production(↑BMR) Hypothyroid patient → hypersensitive to cold Hyperthyroid patient → hypersensitive to heat Hypothyroid patients are prone to develop hypothermia perioperatively. Hyperthyroid patients are prone to develop thyroid storm perioperatively

20. On Protein metabolism : In physiological doses, T4 is anabolic → ↑Protein synthesis Effect of Hyperthyroidism on protein metabolism ↑BMR → ↑protein catabolism → - ve nitrogen balance ↓ ↓ Body Weight Thyrotoxic Osteoporosis myopathy Effect of Hypothyroidism on protein metabolism ↓ Catabolism → Deposition of skin proteins(MPS, Hyaluronic acid, Chondrotin sulphate) under skin ↓ Dry, coarse, puffy appearance ↓ Myxoedema

21. On carbohydrate metabolism In physiological doses T₄ produces two opposite effects which balance each other ↑ ed peripheral utilization→ Hypoglycemia ↑ ed glucose absorption, glycogenolysis , gluconeogenesis , ↓ ed insulin secretion → Hyperglycemia Hypothyroid pts are prone to develop Hypoglycemia Hyperthyroid pts/ pts on pharmacological doses : Hyperglycemia

22. On Lipid Metabolism Cholesterol : T₄ has two opposing effects (↑ ed synthesis & ↑ed breakdown), but ↑ed breakdown predominates. ↑ Formation of LDL receptors in liver, resulting in increased hepatic clearance Therefore, T₄ → Hypocholesterolemia Lipids : T4 stimulates lipid synthesis & also leads to mobilization & lipid degradation by stimulating lipases. Two opposing effects, but 2 is more, so predominately, T₄ decreases stores of TG and phospholipids.

23. On Bone Marrow Metabolism T₄ deficiency ↓ ↓Bone marrow metabolism ↓ Absorption of vit B12 ↓ Erythropoesis Anaemia T₄ excess ↑ Erythropoesis O2 release to tissues ↑ 2,3 DPG Hypothyroid patients may have microcytic hypochromic anemia and/or macrocytic anemia

24. On vitamins Thyroid hormones ↑Metabolic process ↑ demand for coenzymes & vitamins from which they are formed Hyperthyroid pts. Vit B-complex & vit C def. Β -carotene Vit -A Retinene T₄ T₄ T₄ deficiency Carotenemia (yellowish discoloration to skin only)

25. Growth and development Thyroid hormones are essential for normal growth and skeletal maturation as It increases protein synthesis and Potentiates the action of GH In Hypothyroidism – slow growth and epiphyseal closure is delayed Thyroid hormone deficiency leads to cretinism in children and myxoedema in adults

26. Effects on Cardiovascular System ↑ed catecholamine sensitivity and Beta- adrenergic receptor number in heart, skeletal muscles, adipose cells and ↓ed cardiac alpha adrenergic receptors Combined of T4 & catecholamines is ↑ed sympathetic activity on β -receptors via c-AMP ↑ HR, ↑ myocardial contractility, ↑SBP & ↑ CO T4 → vasodilatation → warm & moist skin ↓ PR → ↓ DBP ↑ SBP with ↓ DBP & mild ↓ Mean BP & ↑ PP

27. hypothyroid patient has hypodynamic and hyperthyroid patient has hyperdynamic circulation. Prone to develop High Output Cardiac failure ↓ in Circulation time → ↑ Velocity of blood flow Inadequate tissue perfusion ↑ in Myocardial O₂ consumption is far greater than the ↑ Coronary blood flow → Cardiac Arrythmias

28. On Nervous System CNS T₄ → Normal development & activity of CNS T₄ deficiency after birth → Defective Myelination, ↓ Vascular bed → Infantile brain (Mental retardation) T₄ deficiency in adults → Loss of intellectual function, memory → Myxoedema Madness T₄ Excess in adults → Stimulates RAS → Anxiety, Emotional Instability, Insomnia, Overexcitability PNS T₄ stimulate muscle spindle→ Contraction of muscles T₄ deficiency → Decreases conduction of impulses in nerve fibres & ↓ rate of muscle contraction & relaxation causing prolonged reaction time Myxoedema → Knee jerk time increases Thyrotoxicosis → Decreases

29. On GIT T₄ deficiency : Constipation ↓ Intestinal motility ↓ Food Intake T₄ Excess : Diarrhoea due to ↑ Intestinal motility ↑ Gastric emptying ↑ Intestinal transit time On Respiratory system Maintains normal hypoxic and hypercapnic drive in respiratory centre.

30. On Gonads Hypothyroid : Poor gonadal development in Cretins, Menorrhagia Hyperthyroid : Oligomenorrhoea Essential for maintenance of endometrial secretory phase and fertility Alteration of Thyroid activity → Impaired fertility

31. Thyroid in pregnancy Enlarges by 50-70%, follicular hyperplasia and ↑ vascularity Iodine requirements increases ↑ in TBG → ↑ in total T3 and T4 concentrations but free T3 and T4 don’t change TSH ↓ during first trimester due to the thyrotrophic effect of hCG

32. Thyroid Function Tests

33. Thyroid function tests : normal values Hormone SI units Metric units Total T4 60 -140 nmol /l 5 -11 µg/dl Free T4 10 – 25 pmol/l 0.7 – 2.1 ng /dl Total T3 1.1 – 2.7 nmol/l 80 – 200 ng /dl Free T3 3 – 8 pmol/l 0.2 – 0.5 ng/dl Reverse T3 0.2 – 0.7 nmol/l 15 – 45 ng /dl TSH 1 – 18 pmol/l 0.3 – 5 mU /l

34. Measurement of thyroid hormones Serum TSH Best, initial test to asses the thyroid function Central to the negative-feedback system 2 subunits : α & β TSH α is identical to α subunit of LH, FSH and hCG Normal secretion rate- 110 µg/day Plasma level- 2 mU /L (0.3-5 mU /L) Plasma half life- 60 min TSH (0.1-0.4 mU /L) → Subclinical Hyperthyroidism Suppressed TSH (<0.03 mU /L)→ Overt Hyperthyroidism Suppressed TSH (<0.01 mU /L)→Thyroid storm Elevated TSH (5- 10 mU /L) → Subclinical Hypothyroidism Elevated TSH (≥20 mU /L) → Overt Hypothyroidism

35. Immunochemiluminometric (ICMA) assays are highly sensitive to detect minimal changes in TSH TSH should not be used to assess thyroid function in patients with suspected or known pituitary disease Low TSH level in a clinically hypothyroid patient indicates disease at pituitary or hypothalamic level. In tumors of placental origin, ↑ hCG can activate thyroid receptors nonspecifically and can cause mild hyperthyroidism. Normalization of TSH is the end point of Thyroxine replacement

36. Serum T₄ and T ₃ Total serum T₄ & T₃ assays are added after TSH levels to confirm the diagnosis.. Measurements Total T₄ & T₃ assays – Radioimmunoassay (RIA) Serum T₄ levels are influenced by THBG levels so T₄ levels should never be used alone for evaluation Serum T₃ levels are often determined to detect disease in patients with clinical evidence of hyperthyroidism in absence of raised T₄ levels.

37. Free T₄ & T₃- Directly by Equilibrium dialysis techniques or Indirectly by calculation of Free- Thyroxine index (FT₄I/FT₃I) Test for measuring thyroid hormone binding How to calculate Free T₄ / T₃ index (FT₄I/F T₃I)? Free T4 index (FT₄I) = Total T₄ x THBR (Thyroid hormone binding ratio) THBR is derived from T₃ resin uptake test FT₄I levels are usually identical to that of Total T₄ and T ₃ as THBR is normally 1

38. T₃ resin uptake test T₃ resin uptake test determines the distribution of radiolabeled T₃ between an absorbent resin and unoccupied thyroid hormone binding protein. Binding of radiolabeled T₃ to resin is: ↑ ed when total T₄ in sample is ↑ ed or unoccupied protein binding sites is ↓ed (TBG def.) ↓ ed when total T₄ in sample is ↓ ed or unoccupied protein binding sites is increased. When TSH elevated→ Free T₄ measured to distinguish between overt & subclinical hypothyroidism. When TSH suppressed or subnormal → Both Free T₄ & T₃ measured to distinguish subclinical & overt thyrotoxicosis & to identify T₃ toxicosis.

39. Tests of thyroid gland dysfunction Condition T 4 THBR T3 TSH Hyperthyroidism Increased Increased Increased Normal/low Primary hypothyroidism Decreased Decreased Decreased Increased Secondary hypothyroidism Decreased Decreased Decreased Decreased Sick euthyroid syndrome Normal Normal Decreased Normal Subclinical hypothyroidism Normal Normal Normal Increased Pregnancy Increased Decreased Normal Normal

40. Radioactive Iodine uptake The oral administration of radioactive (I¹³¹) can be used to indicate thyroid gland activity Normal range : 10-25% Uptake is elevated in hyperthyroidism unless the hyperthyroidism is caused by thyroiditis Radioactive iodine uptake may be increased by variety of factors i.e. dietary iodine deficiency, renal failure, CHF Graves’ disease ----> ↑ uptake homogenously Toxic adenoma -----> Focal areas of ↑ed uptake Toxic MNG ----- >Multiple areas of ↑ed or ↓ed tracer uptake Subacute thyroiditis, T hyrotoxicosis factitia ----> Low uptake

41. Thyroid scan Delineates the active thyroid tissue using I123 or Tc99 evaluate nodules as Warm/normal Hot/ hyperfunctioning – rarely malignant Cold/ hypofunctioning – malignant/ benign

42. Tests to determine etiology of Thyroid dysfunction Serum Thyroglobulin ( Tg ) – Tg produced and released by thyroid gland. - ↑ ed levels - all types of Thyrotoxicosis except Factitious Thyrotoxicosis - Marker for recurrent thyroid cancer Serum Thyroid-Stimulating Immunoglobulin (TSI)- - ↑ ed levels - Graves’ disease (stimulate the TSH-R) - High maternal levels in last trimester predicts Neonatal thyrotoxicosis Antithyroid peroxidase antibodies (Anti TPO) - Specific and sensitive. - Hashimoto’s thyroiditis (in 100% cases) - Graves’ disease (in 50% to 90% cases) - In subclinical hypothyroidism, its presence indicates 4-5 fold increase in chance of overt hypothyroidism

43. Appropriate uses of thyroid tests…. Purpose Test Comment Screening Serum TSH (sensitive assay) Free T4 Most sensitive test for primary hypothyroidism and hyperthyroidism Excellent test Hypothyroidism Serum TSH Antithyroglobulin and antithyroid peroxidase antibodies High in primary and low in secondary hypothyroidism Elevated in Hashimoto’ thyroiditis Hyperthyroidism Serum TSH (sensitive assay) T3 (RIA) 123I uptake and scan Antithyroglobulin and antimicrosomal antibodies TSH receptor antibody (TSH-R Ab [ stim ]) Suppressed except in TSH-secreting pituitary tumour Elevated Increased diffuse versus ‘hot’ areas Elevated in Graves’ disease Usually positive in Graves’ disease Nodules FNAC I¹²³ uptake & scan 99mTc scan USG Best diagnostic method for thyroid cancer Cancer is usually ‘cold’; less reliable than FNA Vascular versus avascular Solid vs Cystic; pure cysts are usually not malignant

44. Take Home Message Thyroid hormones exerts variety of action by regulating the synthesis and activity of various proteins Thyroid hormones are not absolutely necessary but required for proper cardiac, pulmonary and neurologic functions T₃ is physiologically active and 80% is produced by extrathyroidal deiodination of T₄ Less than 0.1 % of T₃ is present as active or unbound form TSH is the first test in evaluating suspected thyroid dysfunction Some measures of free thyroid hormone activity (Free T₄ ) must be used as total T₄ levels can be misleading..

45. Serum T₃ levels are often determined to detect disease in patients with clinical evidence of hyperthyroidism in absence of T₄ elevation (to rule out T₃ toxicosis). TFT must be repeated only after 6 weeks on steady dose. Thyroid hormones are important for regulation of thermogenesis. Thyroid hormone increases beta receptors and decreases alpha adrenergic receptors. In ICU, Free T4 and T3 should be performed due to variation in plasma protein levels in critically ill patients. Although the thyroid hormone is important to many aspects of growth and function, the anaesthesiologists is most often concerned about the cardiovascular manifestation.

46. References Harrison’s Principles of Internal Medicine, 19 th ed. Ganong’s Review of Medical Physiology, 23 rd ed. Miller’s Anesthesia , 9 th ed. Anesthesia & Co-Existing Diseases, Stoelting , 7 th ed. Stoelting’s Pharmacology & physiology in Anesthetic practice 5 th ed. Clinical Anesthesia , Barash, 7 th ed. William’s Text Book of Endocrinology, 12 th ed. Chestnut’s Obstetric Anesthesia

47. Thank you