Clinical Biochemistry - GP - Thyroid Functions Tests (TFT)

Thyroxine is a key hormone in metabolism. Control of its secretion is within a negative feedback loop whereby TSH secreted from the pituitary causes the release of thyroxine (measured as free thyroxine - fT4) which then feeds back to the pituitary to reduce TSH secretion. Hence if the pituitary gland is working OK (which it is in the vast majority of people) then measurement of TSH only will give an indication of thyroid function. fT4 will only be measured if TSH is outside the normal range or the clinical details supplied warrant a fT4 measurement. fT3 measurement is very rarely required.

Please supply clinical details with all requests for endocrine testing to ensure the correct tests are done for the patient.

When to check TFT

1.    Newborn screening - TSH check for congenital hypothyroidism
2.    Symptoms of thyroid dysfunction eg tiredness, anxiety
3.    Any patient with a goitre
4.    Patients on thyroid medication - thyroxine and anti-thyroid drugs
5.    Patients who have had thyroid surgery / radioiodine / neck irradiation
6.    Patients on amiodarone or lithium - both drugs may cause thyroid dysfunction
7.    Atrial fibrillation - 5-10% of patients with hyperthyroidism have AF
8.    Osteoporosis - hyperthyroisism may cause osteoporosis
9.    Dyslipideamia - hypothyroidism may cause elevated cholesterol
10. Abnormal menstruation - may be associated with thyroid disease
11. Subfertility - may be associated with thyroid disease
12. Diabetes - annual review of type 1 and checked at diagnosis with type 2
13. Past history of post partum thyroiditis
14. Patients with Downs or Turners Syndrome

The interpretation of TFT results is best understood by some common thyroid scenarios. Please also see the flowchart. Guidance based on the UK Guidelines for the Use of Thyroid Function Tests, July 2006. Please contact Dr Rob Lord to discuss any cases.

Normal ranges for adults

TSH = 0.38 - 5.5 mIU/L
fT4    = 10 - 18.7 pmol/L
fT3    = 3.5 - 6.5 pmol/L

1. Patient feeling tired. TFT requested. TSH = 1.73 mIU/L.

For the majority of patients this will indicate that the patient is euthyroid and a different cause of tiredness should be sought. However, very rarely the patient may have pituitary disease and so if no other cause apparent then either contact Dr Rob Lord to arrange for free T4 analysis if sample still available in the lab (samples kept for 1 week) or request with next blood test with the form marked for the attention of Dr Rob Lord.

2. Patient feeling tired. TFT requested. TSH = 4.85 mIU/L.

For the majority of patients this will indicate that the patient is euthyroid and a different cause of tiredness should be sought. However, it is known that patients with a TSH result greater than 2 mIU/L are more likely to go on to be overtly hypothyroid in the future (especially if thyroid antibodies - TPO - are elevated). If this is the case then if TSH remains consistently above 2 mIU/L consider possible use of low dose thyroxine - in particular if patient also has other features that may be due to thyroid dysfunction eg raised lipids.

3. Patient feeling tired. TFT requested. TSH = 8.26 mIU/L, fT4 = 13.3 pmol/L.

Assuming the patient isn't on any thyroid medication then these results could indicate subclinical hypothyroidism with possible progression to overt hypothyroidism. The results could also just indicate a temporary elevation in TSH which then returns to normal. So, recheck TFT in 6-8/52. If the TSH is still elevated then request measurement of thyroid antibodies (TPO). Consider thyroxine therapy if TPO positive, patient symptomatic for hypothyroidism or has another disease associated with hypothyroidism eg raised lipids, infertility. If the TSH is > 10 mU/L and fT4 normal or low then treatment with thyroxine is recommended.

4. Patient on thyroxine. TFT requested. TSH = 1.2 mIU/L.

This result indicates that the patient is probably on the correct dose of thyroxine. Aim to titrate therapy so that TSH is towards the lower end of the normal range (0.1-2.0 mIU/L). If the TSH goes below 0.1 mIU/L then this could indicate possible over treatment with increased risk of atrial fibrillation and bone loss in post menopausal women. Some patients request an increase in their dose of thyroxine even when the TSH is at the lower end of the normal range. If the dose is increased then also check fT3 to ensure that this is still in the normal range. Note that if the patient is being treated with thyroxine for secondary hypothyroidism (pituitary disease) then monitor thyroxine replacment with fT4 and not TSH.

5. Patient on thyroxine. TFT requested. TSH = 10.3 mIU/L, fT4 = 13.6 pmol/L

When a patient is first started on thyroxine it may take several months for the TSH to normalise so this result could indicate this. Alternatively if the patient has been taking the thyroxine for sometime and the TSH was previously ok then this could indicate a compliance issue. In this case the TSH is a bit like the HbA1c of diabetes with the fT4 being like glucose. The TSH indicates that over time the patient has not been taking the thyroxine but the fT4 indictaes the taking of thyroxine just prior to the blood test.

6. Patient feeling anxious. TFT requested. TSH = 0.13 mIU/L, fT4 = 15.7 pmol/L.

The TSH is below the lower limit of the reference range. Illness not due to thyroid disease (non thyroidal illness) can lower the TSH and this kind of TFT result is very common in a secondary care setting. The result could also indicate subclinical hyperthyroidism with a possible progression to overt hyperthyroidism.
Some drugs may also lower the TSH. Recheck TFT in 6-8/52. If the TSH is still low then check thyroid antibodies (TPO). Elevated TPO will indicate increased likelihood for progression to overt hyperthyroidism. Very rarely a low TSH with fT4 in the normal range may indicate possible T3 toxicosis. If the low TSH is persistent and no other cuases apparent then request fT3 measurement - with the request marked for the attention of Dr Rob Lord.

7. Patient on carbimazole. TFT requested. TSH = 0.01, fT4 = 16.5 pmol/L.

The fT4 is ok but the TSH is still low. This result may be seen early on in the course of treatment with carbimazole as the TSH may take several months to return to normal. The thyroid axis is best monitored with fT4 (aiming to normailse it) during this stage and then with TSH once this has responded and returned into the normal range. NB Check FBC if patient reports any symptoms of possible infection eg sore throat.

8. Patient pregnant. TSH = 0.23 mIU/L, fT4 = 19.0 pmol/L.

The hCG molecule produced during pregnancy is similar to the TSH molecule. Hence in the first trimester of pregancy there may be a rise in fT4 with a small fall in TSH. Later in pregnancy the fT4 falls with the TSH rising.

9. Patient pregnant. On thyroxine. TSH = 5.03 mIU/L, fT4 = 19.0 pmol/L.

For the first trimester the foetus is entirley dependent upon maternal thyroxine. The maternal thyroxine crosses the placenta and is converted to T3 in the foetal brain. Brain development is highly dependent upon T3. Measurement of TSH is not useful in this situation as it responds too slowly to changes in thyroxine. Hence fT4 must be measured to assess thyroid status in pregnancy.

10. Patient on amiodarone. TFT requested. TSH = 4.13 mIU/L, fT4 = 21.5 pmo/L.

Amiodarone is an antiarrhythmic drug which can have multiple effects on the thyroid axis. Most patients remain clinically euthyroid but may have the above pattern of TFT results (both TSH and fT4 are measured if 'amiodarone' is noted as a clinical detail with the request). This is due to the fact that amiodarone inhibits the conversion of T4 to T3. Amiodarone may also cause hypo and hyperthyroidism and these cases should be referred to an Endocrinologist for assessment and management.