1 / 27

Alison Wong Meme Phung Zhi Yuan Quek

THYROID FUNCTION TESTS. CASE D. Alison Wong Meme Phung Zhi Yuan Quek. CASE. Mr. AR, aged 55 years Recently been prescribed amiodarone as treatment for atrial tachyarrhythmia Medications on a regular basis: - Digoxin 62.5 mcg 3 mane - Warfarin 4 mg d as directed. AMIODARONE.

shakti
Download Presentation

Alison Wong Meme Phung Zhi Yuan Quek

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. THYROID FUNCTION TESTS CASE D Alison Wong Meme Phung Zhi Yuan Quek

  2. CASE • Mr. AR, aged 55 years • Recently been prescribed amiodarone as treatment for atrial tachyarrhythmia • Medications on a regular basis: - Digoxin 62.5 mcg 3 mane - Warfarin 4 mg d as directed

  3. AMIODARONE • Remarkable structural resemblance to thyroid hormones • Iodine-rich benzofuran derivative • 37% by weight is organic iodine

  4. EFFECTS OF AMIODARONE ON THYROID FUNCTION • Effects on thyroid physiology •  peripheral 5’-deiodination of thyroxine (T4)to triiodothyronine (T3) •  in the serum levels of T4 and rT3 and a  in the serum levels of T3 • entry of T4 and T3 into peripheral tissue inhibited

  5. early phase of treatment, thyroid-stimulating hormone (TSH) levels also  in response to the  T3 levels • as treatment continues, the serum TSH levels often fall back to normal as the total T4 concentration rises

  6. Changes in thyroid function test results with development of hyperthyroidism and hypothyroidism

  7. Amiodarone-induced thyrotoxicosis • excessive synthesis of thyroid hormone induced by iodine • excessive  in T4 level alone may not be diagnostic • diagnosis straightforward if T3 or FT3 and TSH suppressed • onset of thyrotoxicosis   in T4 levels,  in T3 levels and a dramatic  in TSH levels

  8. suppression of TSH levels possible in subclinical episodes • single abnormal TSH insufficient • subclinical episodes represented by suppressed TSH, T3 & FT3 at lower end of reference range,  T4 or FT4

  9. Amiodarone-induced hypothyroidism • iodide released by amiodarone metabolism inhibits thyroid hormone biosynthesis and release • subclinical hypothyroidism  moderately  TSH levels • additional tests needed • diagnosis confirmed by  TSH with low T4 or FT4 • low T3 or FT3 unreliable indicator

  10. CONCERNS • Digoxin and Amiodarone - digoxin toxicity • Warfarin and Amiodarone - increased risk of bleeding

  11. THYROID BINDING GLOBULIN (TBG) • Analytical protocols for measurement of TBG • Clinical applications of measuring TBG in evaluating thyroid function.

  12. T3 and T4 • Poorly soluble in plasma • Transported primarily by TBG • TBG = a plasma protein that carries 70-75% of circulating T4 and T3. • Remaining 25-30% of T4 transported by albumin and prealbumin. • T3 no affinity for prealbumin and circulates only with TBG and albumin.

  13. Thyroid Hormone Transport Thyroid binding Globulin Bound T3 and T4 Serum Less than 1% Cell Equilibrium between bound and free hormone exists

  14. Bound T3 inactive • Free T3  active • Bound and free hormone exist in equilibrium

  15. T4 higher affinity for TBG in plasma than T3.  99.95% bound, ~0.05% free • Total T4 is a direct measure of T4 • Estimate of total T4 • approximated by amount of bound thyroxine Under conditions with normal TBG concentrations, the total T4 level reflects the functional state of the thyroid.

  16. PROBLEMS WITH TBG MEASUREMENTS • Changes in TBG levels :  will dramatically alter the total T4 concentration, without affecting the unbound freeT4 level. do not reflect a change in thyroid status equilibrium between the free and bound hormones is maintained • remain euthyroid by the normal free thyroid hormone and TSH concentrations • patient may be physiologically normal but have an abnormal total serum T4 level

  17. There are several conditions that can affect ( or ) TBG levels.

  18. CONDITIONS ASSOCIATED WITH INCREASED TBG SYNTHESIS AND PLASMA LEVELS • Oestrogen effects – pregnancy, oral contraceptives • Infectious Hepatitis • Biliary cirrhosis • Genetic determination - increased genetically by an X-linked abnormality

  19. CONDITIONS ASSOCIATED WITH DECREASED TBG SYNTHESIS AND PLASMA LEVELS • Severe liver disease • Androgens and anabolic steroids • High doses of glucocorticoids • Large doses of phenytoin, aspirin and their derivatives • Nephrotic syndrome • Genetic determination

  20. T3 RESIN UPTAKE (T3RU) • Measures the amount of unsaturated binding sites on the thyroid hormone transport proteins • Provides an indirect estimate of the binding capacity of the plasma thyroid-binding proteins

  21. Proportion of labelled T3 will bind to available sites on the serum TBG; excess will bind to the resin • resin uptake is inversely proportional to the total TBG • 80% sensitivity for hyperthyroidism • 50-60% sensitivity for hypothyroidism

  22. METHODOLOGY OF THE T3 UPTAKE TEST

  23. T3U VALUES • %T3U = camp/cpmR X %T3UR • Typical values for %T3U are 25-30% • Increased Values = Hyperthyroidism • Decreased Values = Hypothyroidism

  24. FREE THYROXINE INDEX (FTI) • Is a reflection of the amount of free T4 • A calculated value correct for changes in TBG concentrations • FTI = (Total T4) X (T3 Resin Uptake) (T4 RU Control)

  25. FTI • Normal FTI: euthyroid subjects, pregnancy, women taking estrogens, patient with nephrosis or hepatitis and persons taking drugs that elevate T3U • Increased FTI: hyperthyroidism • Decreased FTI: hypothyroidism

More Related