Thyroid Function Tests

Case Study B

Presented by:Owen Naidoo

Abdullah OsmanChristine Tanzil

Ayse Togac

Ms MA (a middle-aged woman) presents to A&E with:

- History of abdominal pain- Vomiting

- Features of sepsis.

Investigations resulted in the diagnosis of:

Ruptured appendix (surgically removed)

Peritonitis

Her post-op recovery was complicated by:

Transient oliguric renal failure

Pneumonia

Her TFT results 6 days post-op were as follows:

FT4 5pmol/L 10-25pmol/L

FT3 <1pmol/L 3-8pmol/L

TSH 6 mU/L 0.4-4mU/L

She did not have clinical symptoms of thyroid dysfunction or a goitre and so

throxine treatment was not commenced.

Thyroxine

Two weeks post-op her TFT results were as follows and still she displayed no clinical signs

of thyroid disorder:

FT4 8pmol/L 10-25pmol/L

TSH 11mU/L 0.4-4mU/L

After a further three weeks, her TFT results were as

follows:

FT4 11pmol/L 10-25pmol/L

TSH 7mU/L 0.4-4mU/L

She also had a strongly positive peroxidase antibody

thyroxine treatment was commenced.

Some definitions… Thyroid Stimulating Hormone (TSH)

The levels of TSH are controlled by the pituitary gland depending on the circulating levels of thyroxine

Synthesises thyroid hormones from iodide and tyrosine residues

Thyroid hormones: These hormones are released into the peripheral

circulation when required through a negative feedback system

T4: is broken down in the peripheral circulation into T3 and r-T3

T3 and r-T3 should be found in the same proportions

What is hypothyroidism?

Hypothyroidism occurs in patients where there are insufficient levels of thyroid hormones

There are two types: Congenital hypothyroidism Acquired hypothyroidism

Congenital hypothyroidism

Primary hypothyroidism This is the most common cause of congenital

hypothyroidism

Secondary hypothyroidism This occurs when the pituitary gland produces

insufficient amounts of TSH (thyroid stimulating hormone)

Acquired hypothyroidism

Acquired hypothyroidism is caused by the thyroid gland being damaged (e.g. cancer therapy etc)

Primary hypothyroidism The most common cause is Hashimoto’s disease

Secondary hypothyroidism Is caused by a pituitary tumour

Tertiary hypothyroidism Is caused by a hypothalmic tumour

Signs & Symptoms…

Weakness/ Lethargy/ Slowness Cold intolerance Memory loss Weight gain Dry skin Change in voice (deepening) Mild anaemia/pernicious anaemia Effusions: pericardial, pleural, peritoneal &

joint spaces

PART 2

TASKS

ANALYSE MA’S RESULTS

ARE THEY CONSISTENT WITH HYPOTHYROIDISM?

MRS MA’S TFT RESULTS

TFT 6 DAYS POST OPP

14 DAYS POST OPP

35 DAYS POST OPP

REFERENCE

FT3 <1     3-8 pmol/L

FT4 5 8 11 10-25 pmol/L

TSH 6 11 7 0.4-4 mU/L

 At 35 days tested strongly positive peroxidase antibody thyroxine commenced

SUMMARY OF MA’S TFT’S POST OP

DAY 6 Decreased FT4, elevated TSH

Day 14 Decreased FT4, elevated TSH

Day 35 Normal FT4, elevated TSH

GRAPH SHOWING RELATIONSHIP OF MA’S TSH TO FT4

4

5

6

7

8

9

10

11

12

0 5 10 15 20 25 30 35 40

DAYS POST OP

FT4 (pM)

TSH (mU/L)

FSH (0.4-4 mU/L) FT4 (10-25 pmol/L)

DIAGNOSING MRS MA

WHAT DO WE KNOW: MA does have elevated TSH suggests hypothyroidism Most common cause of HT in the US is Hashimoto’s

Disease. MA is middle aged female likely candidate for

Hashimoto’s Strongly positive peroxidase antibody (an

autoantibody) Positive autoantibodies in 95% of cases of Hashimoto’s Advanced Hashimoto’s: low FT4, high TSH MA low FT4, elevated TSH BUT later normal FT4.

GRAPH SHOWING RELATIONSHIP OF MA’S TSH TO FT4

4

5

6

7

8

9

10

11

12

0 5 10 15 20 25 30 35 40

DAYS POST OP

FT4 (pM)

TSH (mU/L)

FSH (0.4-4 mU/L) FT4 (10-25 pmol/L)

POSSIBLITIES FOR MRS MA

READING TIME TFT RESULTS POSSIBLE DISEASE STATE

Possibly initially Low FT4, low TSH NTI (sepsis etc.)(aka euthyroid sick syndrome) 

6 DAYS PO Low FT4, high TSH Recovery phase of NTI, advanced Hashimoto’s 

12 DAYS PO Low FT4, high TSH Recovery phase of NTI, advanced Hashimoto’s 

35 DAYS PO Normal FT4, high TSH Subclinical hypothyroidism, recovery phase of NTI,

CONCLUSION +ve antibody strongly suggests

Hashimoto’s But why the drastic changes in TFT’s? ?MA has a subclinical hypothyroidism

disorder (typical of early Hashimoto’s disease)

Unknown to patient since asymptomatic. During her illness she suffered from NTI

which decreased her FSH and T4 On recovering, levels return to her regular

levels of subclinical hypothyroidism.

TO TREAT OR NOT TO TREAT

During stay at hospital (NTI) controversial

Are patients TSH values decreasing and T4 values increasing?

If yes no need to treat. If no controversial but guidelines

recommend: treat if TSH>10mU/L or if TSH 5-10 mU/L and +ve peroxidase Ab &/

goitre (AACE, 2002).

WHY DO WE TREAT EVEN IF ASYMPTOMATIC

Avoid progression to overt hypothyroidism (3-20%, increased if +ve Ab)

Decrease CV effects, dyslipidemia, neuropsychiatric events

The Results…

6 days post-op 2 wks post-op 5 wks post-op Normal Range

T45 8 11 10-25 pmol/L

T3< 1 3 – 8 pmol/L

TSH 6 11 7 0.4 – 4 mU/L

What are the likely explanations for this series of TFT results?

Hashimoto’s Thyroiditis Subclinical Hypothyroidism Euthyroid Sick Syndrome (aka

NTI)

1. Hashimoto’s Thyroiditis Refers to autoimmune disorders of the thyroid gland. Antibodies and WBC’s damage the thyroid gland Due to excess WBC’s and fluid in the thyroid gland a

‘goitre’ is produced, leading to destruction of thyroid cells & HYPOTHYROIDISM

Destruction of thyroid gland decreases T4 production and as a results TSH increases which makes the goiter even larger.

In this condition thyroid antibodies and usually low; however this is not the case for Ms MA (‘…..strongly positive peroxidase antibody’)

2. Subclinical Hypothyroidism

Scenario characterised by a normal serum T4 and moderately high TSH levels. (N.B T3 levels are usually normal and thus don’t provide much extra in terms of diagnosis)

Serum antithyroid antibodies against peroxidase are usually, but not always, positive (as witnessed by Ms MA)

3. Euthyroid Sick Syndrome

Situation whereby patients with other non-thyroidal illnesses may have abnormal TFTs, mainly because of decreased peripheral conversion of T4 to T3 and decreased binding to TBG.

Clinical features of Euthyroid Sick Syndrome include low T3; normal or low T4 and variable TSH.

Describe analytical principles behind free T4

measurement…

What is free T4?

Free T4 or thyroxine is unbound and hence biologically active and responsible for the regulation of thyroid function through the pituitary feedback mechanism. Besides being a more specific indicator of thyroid function than total T4, free T4 is not subject to the spontaneous fluctuations or drug-induced changes that occur with total T4.

Principles of free T4 measurement…

Principle methods for measuring free T4 is as follows:

Equilibrium Dialysis (ED) Equilibrium Dialysis:-FT4 measured

directly by a sensitive RIA in the dialysate

Ultrafiltration Direct Immunoassays Free T4 index method

Free T4 determination by Equilibrium Dialysis

The serum is put inside the cylinder where bound is separated from free hormone.

The gold standard for measuring free T4 is overnight equilibrium dialysis of serum containing 125I-T4. -The percentage of free T4 is calculated by determining the total counts in the dialysate divided by the total 125I-T4 added to the serum multiplied by the total T4 concentration

Free T4 determination by Ultrafiltration

Ultrafiltration has almost the same principle as ED.

The serum has labelled T4 and this is filtered against a protein free buffer.

Free T4 concentration is worked out as: radiolabelled iodine is inversely proportional to free T4 concentration.

Free T4 determination by Immunoassays

There is a one step and two step method for calculating free T4 concentration by immunoassay (IAS).

Step 1 method: This method is based on the assumption that structurally modified and labelled analogues of T4 will not bind to serum thyroid hormone binding proteins but will compete with free T4 for binding to the T4 antibody introduced in the assay.

Free T4 determination by Immunoassays (cont)

Radio-labelled T4 analogue is added to anti-T4 antibody.

The serum is added to the anti-T4 antibody simultaneously.

Competition occurs and both T4 is removed.

Then you measure proportion of labelled T4 that became antibody bound.

Step 2 method:

Free T4 in patient serum is removed by binding to T4 antibody, which is attached to a solid phase.

The serum is then removed. Next, Radio-labelled 125I-T4 is incubated

with the solid phase that has unbound sites available.

Radio-labelled 125I-T4 is removed and activity is quantified.

Free T4 determination by Index method

The index method requires two independent tests.

One measuring total serum T4 and the other measuring thyroid hormone-binding ratio or T3 resin uptake.

The free T4 index is then calculated using the total T4 and the TBG level, the thyroid binding ratio, or T3 resin uptake.

The index is directly proportional to the free T4 level.

Advantages and Disadvantages

Equilibrium Dialysis: Advantages: Gold standard, accurate Disadvantages: Time consuming,

expensive, technically demanding Immunoassay:

Advantages: quick compared to ED, higher accuracy than ED, regularly available

Disadvantages: expensive, procedure has to be carried out precisely

What factors can effect T4?

Age Infection Stress Pregnancy

What Drugs can effect T4?

Amiodarone: structurally resembles thyroxine molecule. Decreases serum T4 levels

Phenytoin and Carbamazepine: accelerate clearance of T4 and depress FT4

Propranolol: elevation of serum free T4 levels Lithium: inhibits T4 release. Glucocorticoids: suppress T4 levels