How to test your thyroid gland

TSH 0.4 – 4.5 mU/L

TSH is used as a screening test for thyroid dysfunction, and also to monitor the doses of treatment for thyroid disorders. The level of TSH reflects the amount of free, biologically-active thyroid hormone present in our body tissues.

TSH is produced by the pituitary gland in the brain, and sends a signal to thyroid gland causing it to release T3 and T4.

TSH levels are low in hyperthyroidism including Graves’ disease as a result of negative feedback signals to the pituitary from excess T3 and T4.

Conversely, they are high in primary hypothyroidism as the pituitary produces more TSH in an effort to raise T3 and T4 levels. In cases of central hypothyroidism, the TSH levels may be normal or low.

A note on reference ranges

Reference ranges allow us to define the values that are ‘normal’ for a healthy population. In the UK the typical reference range for TSH is 0.4-4.5mU/L. The reference range comprises 95% of the population, which means that 95% people who do not have a thyroid illness, will have a TSH between 0.4-4.5mU/L. It is important to note that the reference ranges for different laboratories may vary slightly, as they may use different reference populations.

There is growing controversy about the upper limit of the reference range for TSH.

In the United States, the National Academy of Clinical Biochemists (NACB) have suggested that the upper limit of TSH should be reduced to 2.5 mU/L. This would result in more people with symptoms being classed as hypothyroid, and thereby being able to access treatment.

The British Thyroid Association, recognise that this is a controversial area, however, do not recommend that the reference range should be narrowed. It is not standard practice within the UK to offer levothyroxine to patients who have a TSH between 2.5- 4.5mU/L. Some endocrinologists, however might offer it to a patient with symptoms, particularly if they have thyroid autoantibodies. If you fall within this category and feel that you would benefit from treatment, it is important to discuss this with your doctor. As with any medications, there are risks associated with the inappropriate or unnecessary usage of levothyroxine, particularly in the elderly.

FT4  9.0-25.0 pmol/L

You may read references to ‘free’ T3 and T4. The majority of T3 and T4 is bound to a protein called ‘thyroxine binding globulin’ or ‘TBG’. The bound forms of these hormones are inactive in contrast to the unbound or ‘free’ forms.

Our laboratory measures and reports free T4 levels.

T4 levels are raised in hyperthyroidism and low in hypothyroidism.

FT3 3.5-7.8 pmol/L

T3 makes up 15% of our thyroid hormone. T4, in the remaining 85%, is approximately 5 times less metabolically active than T4. It is T3, therefore, that is mostly responsible for the effect of thyroid hormones on the cells.

You may read references to ‘free’ T3 and T4. The majority of T3 and T4 is bound to a protein called ‘thyroxine binding globulin’ or ‘TBG’. The bound forms of these hormones are inactive in contrast to the unbound or ‘free’ forms.

Our laboratory measures and reports free T3 levels (FT3).

T3 levels are raised in hyperthyroidism and low in hypothyroidism.

The current understanding is that low levels of T3 can be corrected for by increasing the conversion of T4 to T3 at the ‘organ level’ i.e. the organs requiring T3 do this themselves and so can compensate for the low T3.

However, if the process of conversion is abnormal in some way, it is possible that your T4 levels may be normal but your T3 levels abnormal. If there is failure to convert T4 to T3, you may be symptomatic as a result of being low in T3 alone. In order to diagnose a conversion problem, you would therefore need to know free T4 and free T3 levels. Certain medications can also disrupt the conversion of T4 to T3, including amiodarone and steroids.

There is evidence[1] to suggest that in patients already on T3-T4 combination therapies (not generally available on the NHS) there is value in monitoring of T3 levels.

Lastly, T3 can be used in the diagnosis of a rare condition called T3 thyrotoxicosis. In this scenario the patient has symptoms of hyperthyroidism (see above) but their T4 levels are normal. The T3 is raised and the TSH is usually low.

References:

[1] http://www.btf-thyroid.org/images/documents/BTA_Hypothyroidism_Statement.pdf

A note on ‘sick euthyroid syndrome’

Sick euthyroid syndrome is the name for the syndrome that occurs when illness occurring outside of the thyroid gland e.g. having an infection, results in abnormal thyroid function tests.

TSH and free T4 results can be normal, low or high. Free T3 is usually low due to reduced conversion of T4 to T3. These results would be expected to normalise once the patient becomes well.

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Claire is the Inbound and Content Manager here at Thriva. Her mission is break through all the clutter of information that is out there relating to health and bring you informative, easy to digest, actionable insights.