Chapter 5 .2 ThyroidFunction test For medical laboratory

TFT presentations

1. 5.2. Thyroid Function
Anatomy, Physiology &
Disorders

2. Learning Objectives
Upon completion of this lecture the student will be able to:
1.Define terms related to thyroid function
2.Discuss the source and metabolism of thyroid
hormones
3.Explain the clinical significance of thyroid hormone
results
4.Describe methods of analysis of thyroid hormones

3. Learning Objectives
Upon completion of this lecture the student will
be able to:
5. Interpret thyroid hormone results compared to
the reference ranges.
6. Discuss sources of error in thyroid hormone
analysis.

4. Outline of Instruction
Introduction
Source
Clinical Significance
Methods of Analysis
Specimens
Quality Control
Interpretation of Results
Sources of Error
Documentation and Reporting
Summary

5. Introduction to Thyroid
Function
Definition to terms:
Thyroid gland: small tissue situated in the neck just below the
larynx producing hormones that affect metabolism and growth.
Thyroxine: 3, 5, 3’, 5’ –tetraiodothyronine or T4; produced by
the thyroid gland
Triiodotyrosine: 3, 5, 3’- triiodotyrosine, T
3; produced from the
thyroid gland and peripheral tissues
Reverse T3: 3, 3’, 5’ triiodotyrosine; inactive T3
Deiodinazation: biochemical process of removing iodine

6. Location of Thyroid Gland

7. Thyroid Anatomy

8. Thyroid Biochemistry and Metabolic
Pathways
Precursor hormones are:
Monoiodotyrosine (MIT)
Diiodotyrosine (DIT)
Iodine and tyrosine
derived
Circulating thyroid
hormones are
Thyroxine (T4)
Triiodotyrosine (T3)

9. Thyroid Biochemistry and
Metabolic Pathways
Step 1: I
-
trapped in thyroid gland
Step 2: I
-
oxidized to I
2
Step 3: I
2 reacts with tyrosine in thyroid cells 
MIT and DIT
Step 4: 2 DIT T
4; MIT+ DIT  T
3

10. Thyroid Anatomy: Cells
Follicular cells
synthesize
thyroglobulin and
thyroxine and T3
Colloid cells provide
Iodine
Capillaries transfer
hormones to circulation

11. Thyroid Biochemistry and
Metabolic Pathways
Thyroid gland makes T4
Peripheral tissues make T3 from T4
Deiodination
Thyroid makes some reverse T3
Inactive form
Thyroid binding globulin
Binds to majority of T4 and T3
Only free T4 and T3 is active

12. Function of Thyroid Hormones
Releasing
Hormone
Ant. Pit.
Hormone
Thyroid
Hormone
Action
 
TRH
 
TSH T
3
and T
4
 
Increases
metabolic
rate of
cells, ATP
and heat
production,
breakdown
of liver
glycogen,
calorigenic
.

13. Endocrine Control: Thyroid
Function

14. Clinical Significance: Primary
Hyperthyroidism
Disease of the primary organ (thyroid)
Increased Free T4 and total T4 and T3
Decreased TSH
Symptoms:
Increased heart rate and metabolism
Problems with eyes
Many other symptoms

16. Secondary Hyperthyroidism
Disease of the secondary organ (pituitary)
Increased Free T4 and total T4 and T3
Increased TSH
Symptoms:
Increased heart rate and metabolism
Problems with eyes
Many other symptoms

17. Clinical Significance: Primary
Hypothyroidism
Disease of the primary organ (thyroid)
Decreased Free T4 and total T4 and T3
Increased TSH
Symptoms:
Decreased heart rate and metabolism
Failure to thrive and retardation in children
Many other symptoms

18. Clinical Significance:
Secondary Hypothyroidism
Disease of the secondary organ (pituitary)
Decreased Free T4 and total T4 and T3
Decreased TSH (and other pituitary hormones)
Symptoms:
Decreased heart rate and metabolism
Many other symptoms

19. Principles of Methods:
Thyroid Hormone Analysis
Immunoassay Procedures:
Radioimmunoassay (RIA)
Chemilumiscent Immunoassay
Fluorescence Polarization Immunoassay (FPIA)

20. Competitive RIA for Total T
4 or T
3
Method
Determine amount of Ab needed to bind to a
known amount of labeled Ag
–Use predetermined
amounts of labeled
Ag and Ab and add a
sample containing
unlabeled Ag as a
competitor
+ ↔
Prior to Test
Labeled
Ag
+ ↔
Test
+
Patient’s
sample
Labeled
Ag
+

21. Principles of Methods:
Thyroid Analysis
Chemilumiscent Immunoassay
Patient T4 + peroxidase labeled-anti-T4  Ag-Ab-label
complex
Ag-Ab-label complex + luminol –(peroxidase) oxidized
product emitting light
Measured in luminometer
Note: this same procedue may be used for T3
but antibody specifity is to T3.

22. T
3 uptake (T
3U) and Free
Thyroid Index (FTI) by RIA
Patient sample +
125
I-T
3 (incubated)
Resin added to remove excess
125
I-T
3
 radiation is counted in cpm
T
3U is % of
125
I-T
3 taken up by resin
Inversely proportion to free TBG binding sites
High uptake of
125
I-T
3 means low levels of free TBG
%T
3U = (cpm
p /cpm
R) x % T
3U
R
FTI = T
3 uptake X T
4 concentration

23. Method of Analysis for Free T4
or Free T3
Separate free T4 (or T3) from TBP-bound T4 (or
T3) with ultracentrifugation
Filter sample prior to analysis
Proceed with RIA or Chemiluminescent
immunoassay
[FT4 x [TBP]= k x [T4: TBP]

24. Specimens for Thyroid
Analysis
Serum
Heparinized or EDTA plasma
Whole blood from Capillary
Dried blood spot

25. Quality Control
A normal & abnormal quality control sample should
be analyzed along with patient samples, using
Westgard or other quality control rules for
acceptance or rejection of the analytical run.
Assayed known samples
Commercially manufactured (Humastar)
Validate patient results
 Detects analytical errors.

26. Interpretation of Results
Thyroid Hormone Reference Ranges
T
4: adult 5.1 to 11.0 µg/dL(66 -142 nmol/L)
T
3 adult 70 to 200 ng/dL (1.08 to 3.08 nmol/L)
%T
3
U: 25 to 35%.
Free T
4: adult 5.6-11.7 g/dL
Free T
3: adult 80-210 ng/dL
Ranges are method dependent and age adjusted.
Patient results should be compared with the appropriate reference ranges

27. Sources of Error in Thyroid
Analysis
Hemolyzed specimens
Wrong anticoagulant
Lipemic
Turbidity from lipids
Specimen exposed to heat or sunlight
Poorly maintained or poorly calibrated
instrument.

28. Reporting and Documentation
To avoid post-analytic errors,
Report the patient result with :
right name and result
Include reference ranges
Timely manner
QC and patient results should be documented in
logbook and retained in lab

29. Summary
Thyroid hormone metabolism, regulation,
mechanism of action were discussed.
Determination of thyroid hormones (T3, T4, FT4,
FT3, FT4I, TBG and RT3U) including principle of
methods, calculations, specimens, interpretation
of results, sources of error, quality control and
documentation and reporting of results were
discussed.

30. References
Burtis, Carl A., and Ashwood, Edward R.. Tietz:
Fundamentals of Clinical Chemistry. Philadelphia, 2001
Arneson, W and J Brickell: Clinical Chemistry: A
Laboratory Perspective 1
st
ed. 2007 FA Davis