CLINICAL CHEMISTRY CHAPTER 20

1. 4/15/2012 1 CLINICAL CHEMISTRYCHAPTER 20 THYROID FUNCTION

2. 2 Introduction Have you been feeling lousy lately? Always too hot or too cold? Losing weight? Getting fat? Sweating like a pig? Cold as a clam? If you answered yes to any of these questions, then your thyroid gland may be talking to you.

3. 3 KEY TERMS Free Thyroxine Index ( FTI ) Graves� Disease Hashimoto�s Disease Hyperthyroid Hypothyroid Euthyroid Subclinical Thyroid disease T3 Uptake ( T3U ) Thyroid Binding Globulins Thyroxine Binding Globulin (TBG) Thyroxine Binding Albumin ( TBA) Thyroxine ( T4 ) Triiodothyronine ( T3 ) Thyroid Stimulating Hormone (TSH) Thyroxine Hormone Binding Ratio (THBR ) Iodide Hypothalamus Gland Anterior Pituitary Gland Thyroid Gland Free T3 and T4 Bound T3 and T4 TSH Assay �Generation� TSHR and TPO antibodies

4. 4 Objectives Define each of the Key Terms Discuss the synthesis, transport and effects of the thyroid hormones Discuss the feedback systems that regulate thyroid concentrations List common tests that are used to evaluate thyroid function Discuss the common methodologies used to measure thyroid hormones Interpret the results of thyroid testing and relate them to specific thyroid disease conditions

5. 5 Thyroid Anatomy and Physiology 2 lobes gland and the base of the neck Produces Thyroxine ( T4 ) and Triiodothyrine ( T3 ) Iodide is an essential component of T4 and T3 Thyroid gland concentrates iodide from plasma Iodide is oxidized to I0 or I1+ which combines with the glycoprotein thyroglobulin Thyroglobulin contains tyrosyl groups, which react with iodide to form MIT or DIT

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7. 7 Formation of T3 and T4 in the thyroid gland MIT + DIT = T3 DIT + DIT = T4 T4 - Iodide = rT3 ( Reverse T3 ) � Inactive form T3 and T4 are stored and released into the plasma by the enzymatic cleavage of thyroglobulin T4 concentrations are 50 times greater than T3 Peripheral de-iodination of T4 to produce T3 ( in liver & kidney ) T4 � Iodide = T3 ( 80% of T3 )

8. 8 Chemical Structure of T3 and T4

9. 9 Greater than 99 % of circulating T3 and T4 are bound to plasma transport proteins Thyroxine Binding Globulin ( TBG ) Transthyretin ( TTR ) Thyroxine Binding Pealbumin ( TBPA ) Thyroxine Binding Albumin ( TBA ) On average, ? of protein binding sites are occupied by T3 or T4 Percentages of T3 and T4 in the free ( active ) forms 0.4 % of T3 0.04 % of T4 T3 has the greatest hormonal effect

10. 10 ? Binding Proteins ? Bound T3 and T4 ? Binding Proteins ? Bound T3 and T4 Free T3 and T4 remain stable, but Total T3 and T4 may vary This is a problem because clinical conditions that effect the concentrations of the Thyroid Binding Proteins also effect the Total T3 and Total T4 hormones � But the Total T3 and T4 are not the physiologically active forms This is why we have the T3U and THBR tests � to measure what effect the Thyroid Binding Proteins are having on the Total T3 and Total T4 values

11. 11 Causes of elevated Thyroxine Binding Globulins Pregnancy Estrogen therapy and oral contraceptives Hypothyroidism Causes of decreased Thyroxine Binding Globulins Protein malnutrition Cirrhosis Androgens Acute and chronic illnesses Hyperthyroidism

12. 12 Regulation of Thyroid Hormones

13. 13 TSH ( Thyroid Stimulating Hormone ) Glycoprotein hormone Composed of Alpha and Beta subunits Same subunits as LH, FSH and HCG - Possible cross-reactions A 2 fold increase or decrease in T4 results in a 100 fold increase or decrease in TSH. For this reason � TSH is the single best test to determine Primary Hypothyroidism or Hyperthyroidism

14. 14 Roles of the Thyroid Hormones Brain and skeletal development in children Regulates cellular oxygen consumption Regulates heart rate Regulates carbohydrate and lipid metabolism

15. 15 SYMPTOMS OF HYPERTHYROID MOIST WARM SKIN WEIGHT LOSS SWEATING NERVOUSNESS TREMORS HEAT INTOLERANCE TACHYCARDIA

16. 16 SYMPTOMS OF HYPOTHYROID WEAKNESS LETHERGY COLD DRY SKIN, WEIGHT GAIN, SLOW SPEECH ? CHOLESTEROL COLD INTEROLANCE HOARSENESS BRADYCARDIA

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18. 18 COMMON TESTS FOR THYROID FUNCTION TSH TOTAL T3 AND T4 (TT3 AND TT4 ) FREE T3 AND T4 ( FT3 AND FT4 ) T3U ( T3 � UPTAKE ) �..MEASURES BINDING PROTEINS FREE THYROXINE INDEX ( FT4I ) �. ESTIMATE OF FT4 SEROLOGICAL TESTS FOR ANTI-THYROID AUTOANTIBODIES

19. 19 CLASSIFICATION OF THYROID DISORDERS THYROID DISORDERS ( AND OTHER HORMONES TOO ) CAN BE CAUSED BY DIFFERENT GLANDS THAT FORM THE ENDOCRINE SYSTEM PRIMARY : THYROID GLAND DISORDER SECONDARY : ANTERIOR PITUITARY GLAND DISORDER TERTIARY : HYPOTHALAMUS GLAND DISORDER EXAMPLE : A SECONDARY THYROID DISORDER IS ACTUALLY A DYSFUNCTION OF THE ANTERIOR PITUITARY

20. 20 MOST METHODOLOGIES FOR THYROID TESTING UTILIZE A VARIETY OF COMPETETIVE BINDING TECHNIQUES RIA, ELISA, FIA KEEP IN MIND THAT TECHNIQUES THAT MEASURE TOTAL T3 OR TOTAL T4 WILL BE AFFECTED BY THE CONCENTRATIONS OF THE THYROID BINDING PROTEINS FREE T3 AND FREE T4 TESTING REQUIRE SPECIAL TECHNIQUES THAT SEPARATE FREE FROM PROTEIN-BOUND HORMONES ( EQUILIBRIUM DIALYSIS TECHNIQUE ) �GENERATIONS� OF TSH TEST REFERS TO A 10-FOLD INCREASE IN SENSITIVITY

21. 21 T3 � UPTAKE ( T3U ) TEST MISNOMER !!! DOES NOT MEASURE T3 !!! Estimation of the Thyroid Binding proteins VARIATIONS IN THYROID BINDING PROTEINS MAY BE UNRELATED TO THE THYROID GLAND VARIATIONS IN THE BINDING PROTEINS STILL RESULT IN EUTHYROID PATIENTS BECAUSE THE THYROID ADJUSTS SECRETION OF HORMONES SO THAT THE CONCENTRATIONS OF THE FREE HORMONES REMAINS NORMAL

22. 22 T3U ( CONTINUED ) T3U METHODOLOGY PATIENT�S PLASMA INCUBATED WITH A TAGGED T3 EXCESS T3 REMOVED ( BOUND ) TO BINDING AGENT ( RESIN, ANTIBODY, CHARCOAL ) T3 IS MEASURED IN BINDING AGENT AND RESIDUAL PLASMA ? T3 UPTAKE BY BINDING AGENT � ? THYROID PROTEIN SITES ? T3 UPTAKE BY BINDING AGENT � ? THYROID PROTEIN SITES T3U IS REPORTED AS % OF NORMAL AVERAGE VALUE OR THBR INCREASED T3U : 1? HYPERTHYROID, STEROIDS DECREASED T3U : 1? HYPOTHYROID, PREGNANCY

23. 23 Example T3 - Uptake Test ( T3U)

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25. 25 FT4I ( FREE T4 INDEX ) CALCULATED ESTIMATE OF FREE T4 CORRECTS FOR ETHYROID PATIENTS WITH ABNORMAL THYROID BINDING HORMONES CALCULATION FT4I = TT4 X THBR (Thyroxine Hormone Binding Ratio) THBR = The control T3U usually represents the T3U value from a pool of normal patients and by definition has a THBR of 1.0 EXAMPLE: EUTHYROID PATIENTS WITH INCREASED THYROID BINDING PROTEINS WILL HAVE INCREASED TT4 AND FALSELY LABELED AS HYPERTHYROID, BUT WILL HAVE A NORMAL FT4I

26. 26 Drug interference with T3U Different drugs can compete with T3 and T4 for binding sites on the TBGs, causing falsely increased T3U results Dilantin Coumadins Heparins Aspirin

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28. 28 Another Illustration of T3U Test

29. 29 COMMON THYROID DISORDERS GRAVES DISEASE AUTOIMMUNE, COMMON CAUSE ( 80% ) OF HYPERTHYROID CASES, ANTI-THYROID ANTIBODIES HASHIMOTO�S DISEASE COMMON HYPOTHYROIDISM, INFILITRATION OF THYROID GLAND BY LYMPHOCYTES AND ANTI-THYROID ANTIBODIES MYXEDEMA SEVERE HYPOTHYROIDISM NEWBORN SCREENING 1 IN 4,000 BIRTHS HAVE CONGENITAL HYPOTHYROIDISM

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32. 32 TOP 10 T4 = Thyroxin and T3 = Triiodothyrine Iodide is required for thyroid hormone synthesis ( look at your salt ) Hyperthyroid �speeding metabolism / Hypothyroid � slow metabolism� > 99% of thyroid hormones are bound to proteins Only free thyroid hormones are physiologically active TSH is synthesized by the Anterior Pituitary Primary Hypothyroid Increased TSH ( and visa versa ) T3U test measures thyroid binding proteins FT4I = TT4 x THBR Graves� Disease : Autoimmune hyperthyroid condition

33. 33 REFERENCE RANGES: TSH 0.5 - 5.0 ?U / ml T4 4.5 - 13.0 ?g / dl T3 60 - 220 ng / dl T3U 25 - 30% TBHR 0.8 - 1.4

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