THYROID SERIES PART 3 TFTS

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1.      How do we evaluate thyroid disorder?
 


                                                                             


                                                           SERUM THYROID HORMONES

2.      What are the serum thyroid hormones tested?

·         TSH

·         Total and free T4 and T3

·         Thyroglobulin

 

3.      Which is the most sensitive and specific test?

·         TSH

·         Since T4 and TSH have a negative log-linear relationship between serum freeT4 and TSH,very small change in T4 can markedly change TSH.

 

4.      Patients taking biotin should hold the supplements for 2 days prioe of TFTs and longer if they are taking >10mg.

It is because biotin may cause false low values in immunometric assays and false hogh values in competitive binding assays.

 

5.      How do we measure serum TSH?

BY CHEMILUMINOMETRIC ASSAY which can measure value as low as 0.1mU/L.

 

6.      Reference value.

 


  

 

7.      fT3 vs Total T3 and fT4 vs Total T4

free or unbound hormones are biologically active and are not effected by TBG binding which effects total hormones.

Thus free or unbound hormones are measured.

 

8.      How do we measure fT4 and fT3 ?

By direct hormone assays using

·         Unbound thyroid hormone competition with radiolabeled T4 (or an analogue) for binding to a solid-phase antibody, and

·         Physical separation of the unbound hormone fraction by ultracentrifugation or equilibrium dialysis.

 

9.      What are other older methods?

Indirecet methods where free fraction was obtained from total T4 or T3.

 

Free T4 index = total T4 x THBI

 

10.  What is THBI and how is it calculated ?

It is thyroid hormone binding index which obtained by measuring patients radiolabelled T3 uptake.

 

THBI = PATIENTS T3 UPTAKE / normal pool T3 uptake

 

Very cumbersome and complicated thus not used these days.

 

11.  Which test is better fT3 or fT4?

We usually prefer fT4 as fT3 testing is highly unreliable and variable.

But in cases of hyperthyroidism and T3 thyrotoxicosis fT3 is important.

 

12.  What is T3 thyrotoxicocis?

In 2-5% of thyrotoxicosis cases FT4 is normal but TSH is suppressed with elevated fT3/

 

13.  How important is reverse T3 measurement ?

Not useful but rarely used for consumptic hypothyroidism (link) and to distinguish central hypothyroidism from nonthyroidal illness in critically ill patients.

 

14.  When do we do TFTs ?

In cases of  suspision of hypothyroidism and monitoring of levothyroxine therapy.

 

15.  How do we approach a case of suspected hypothyroidism?

If secondary or tertiary hypothyroidism is not suspected serum TSH is sufficient.

 
 

 

    

16.  How do we asses thyroid function in cases of patients receiving levothyroxine ?

By measuring TSH only rather than T4.

 

17.  How do we assess treatment and titrate drug dosing in hyperthyroidism ?

Since TSH level in patient receiving treatment for hyperthyroidism remain low for several weeks to sometimes moths it is better to measure free or total T4 levels.

 

18.  How do we differentiate between TSH mediated(secondary) and TRH mediated(tertiary) Hyperthyroidism?

Though rarely used these days the cumbersome and difficult test called TRFactor (TRH) test.

 

19.  How do we do TRF test?

A dose of 200 mg of TRF is given IV and TSH is measured at 0,10,15,30,60,90,120 and 180 min.

 

Interpretation:

· Normal is increase in TSH to at least 10mIU/mL after about 15 min.

· No increase in secondary hypothyroidism.

· In tertiary hypothyroidism delayed until 60-120 min.

 

20.  What are the thyroid antibody tests?

Anti Thyroglobulin tests (anti- Tg)

Anti TPO (thyroperoxidase) Ab

TRAb (Thyroid Receptor Antibody)

 

21.  How important is Anti-Tg testing?

Not usually done but should be always done when serum thyroglobulin is being tested as ant-Tg Ab can interfere with Tg levels.

 

22.  When is serum thyroglobulin measured?

· In cases of thyrotoxicosis factitia where thyrotoxicosis is caused by self administration of thyroid hormones for weight loss

In cases of all thyrotoxicosis Tg is elevated except factitia

·         After thyroidectomy and radioablation, Tg levels should be undetectable; in the absence of anti-Tg antibodies, measurable levels indicate incomplete ablation or recurrent cancer.

 

23.  How often anti-TPO antibodies are tested ?

Not usually done but  test for anti-TPO antibodies may be useful to predict the likelihood of progression to permanent overt hypothyroidism in patients with subclinical hypothyroidism.

 

24.  Thyrotropin receptor antibodies (TRAbs)

 

·         It  classified as stimulating, blocking, or neutral.

 

·         Stimulating antibodies (thyroid-stimulating immunoglobulins, TSI) cause Graves' disease.

 

·         Thyroid receptor-blocking antibodies can cause hypothyroidism.

 

·         Some patients will have a mixture of both antibodies, and, depending on the relative titers of these antibodies, they may fluctuate between hyperthyroidism and hypothyroidism.

 

·         Neutral antibodies bind the receptor but do not stimulate or block function.

 

·         There are two methods for measuring TRAb:

o   TSI assays measure only thyroid-stimulating antibodies

o   TSH receptor-binding inhibitor immunoglobulin (TBII or TBI) assays measure stimulating, blocking, and neutral antibodies.

 

 

RADIONUCLIDE STUDIES

 

25.  What is the principle of this investigation?

· A radioactive material called a radioisotope, or radionuclide “tracer,” adminstered before the test. The tracer may be administered via an injection, a liquid, or a tablet.

· The tracer releases gamma rays when it’s in the body.

· A gamma camera or scanner can detect this type of energy from outside the body.

·         The camera scans thyroid area. It tracks the tracer and measures how thyroid processes it.

·         The camera works with a computer to create images that detail the thyroid’s structure and function based on how it interacts with the tracer.


26.  What are the commonly used tracers ?

I123 , I131 ,99mTC

27.  How do they differ ?

 

I123

I131

99mTC

Both trapped and organified by thyroid

Both trapped and organified by thyroid

Only trapped not organified

Used in children as less chances of radiation exposure

High radiation exposure

Used in children as less chances of radiation exposure

Half life of 13 hours

half life of 8 days

6 hours

 

Better cytotoxic effecr thus used for radioiodine treatment of graves disease and differentiated thyroid cancer

 

28.  What are the indications for radionuclide imaging?

To identify thyroid dysgenesis

To locate ectopic thyroid

To evaluate possible autonomous nodule.

 

29.  Why IV is better than oral tracer in small children?

 

As small children have propensity to spit the tracer material the gamma camera will falsely show radioactive iodine uptake in skin.

Also children should be fed before imaging to avoid tracer uptake in salivary gland

 

30.  Interpretation of nodules using radionuclide will be discussed later.

 

 

THYROID ULTRASONOGRAPHY

 

 

Uses :

·         Assessing size , location and shape of thyroid gland.

·         Assessing characteristics of thyroid nodules

·         Can also be used to evaluate thyroid dysgenesis

·         Not sensitive for detecting ectopic thyroid

·         Useful for identifying cystic nodules

·         Adjunct for obtaining biopsy

 

Certain characteristics seen in USG can increase the likelihood of malignant nodules like

·         Irregular margins

·         microcalcifications

·         hypoechogenicity

·         taller than wide shape


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 THANK YOU

DR NISCHAL

 


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