Although small, the thyroid gland has many important functions within the human body. Most importantly, it produces and secretes hormones into the blood which influence the metabolism of cells. The thyroid is located at the front of the neck and consists of two lobes, one on each side of the windpipe and adjoined by a small strip of thyroid tissue known as the isthmus.
The three hormones produced by the thyroid are:
- T3 (triiodothyronine)
- T4 (tetraiodothyronine)
T3 and T4 are the active forms of the thyroid hormone, thyroxine, and are involved in the regulation of:
- The body’s metabolic rate
- Heart function
- Digestive function
- Muscle control
- Brain development
- Bone maintenance
What’s the difference between hyperthyroidism and hypothyroidism?
Hyper- and hypothyroidism are conditions which affect the performance of the thyroid.
Hyperthyroidism is where the thyroid is overactive and produces too much of the thyroid hormones.
When too much of T3 and T4 are produced, the following symptoms can occur:
- Feeling nervous, anxious or irritable
- Weight loss
- Mood swings
- Weak muscles
- Low sex drive
- Need to pee more often
- Increased thirst
Hypothyroidism or an underactive thyroid, on the other hand, is characterised by the failure of the thyroid to produce enough T3 or T4. As a result, symptoms can include:
- Feeling cold
- Weight gain
- Muscle cramps and aches
- Low sex drive
- Brittle hair and nails
- Dry skin
- Irregular or heavy periods
How often should a thyroid test be carried out?
A blood test can determine the function of the thyroid. The test measures the levels of TSH, T4 and T3 in the blood. The free portions of T3 and T4 are usually measured as this is the active part of each hormone. Laboratories use reference ranges when analysing the function of the thyroid. Typical reference ranges for a healthy individual may look like this:
- TSH 0.27– 4.2 mIU/L
- FT4 12.0 – 22.0 pmol/L
- FT3 3.1 – 6.8 pmol/L
Different laboratories use different reference ranges, but if your results fall outside of these parameters you could be diagnosed with hypo- or hyperthyroidism. Subclinical hypothyroidism, on the other hand, occurs when your blood TSH level is slightly raised but your FT4 is within the normal range.
How often you will need to have a thyroid test following diagnosis will depend on your condition and treatment. If you are diagnosed with a thyroid condition you will be prescribed treatment with the aim of making you feel better and preventing any long-term damage.
TSH is the most sensitive biomarker for thyroid function and is often used to ensure the hormone replacement therapy you are prescribed is working as effectively as it should.
How often should T3 levels be checked?
Thyroid problems can be identified by measuring the level of FT3 in the blood. High levels indicate an overactive thyroid whereas low levels indicate an underactive thyroid.
FT3 can be used to:
- test to see if your thyroid is performing properly
- diagnose hyperthyroidism
FT3 is only really requested when diagnosing or testing the severity of hyperthyroidism.
How often should T4 levels be checked?
Equally, FT4 is also used to identify thyroid issues and can be used to detect both hypo- and hyperthyroidism. With high levels suggesting an overactive thyroid and low levels of FT4 indicating an underactive thyroid. However, the levels of FT4 are often used in conjunction with presenting symptoms to identify a thyroid problem, rather than alone.
If you are diagnosed with hyperthyroidism, you will usually be offered TSH and FT4 tests, but how often they are performed will depend on the treatment you are receiving.
If hypothyroidism is diagnosed, you will usually begin taking levothyroxine, a synthetic, man-made version of the thyroid hormone, thyroxine. If hyperthyroidism is diagnosed, however, treatment options include:
- Antithyroid drugs
- Surgery to remove some of or all the thyroid gland
- Radioactive iodine to reduce the thyroid’s activity
When treatment commences, it is likely you will have blood tests on a regular basis, possibly every few weeks to assess your body’s response to treatment. Once the treatment has made your condition stable, the frequency of the blood tests should reduce. However, if you have an underactive thyroid you will need to have your TSH levels tested once a year to make sure they are within the normal parameters.
- Blood tests are the most accurate way to diagnose and manage thyroid disorders
- Your symptoms and your general wellbeing are also key to your diagnosis
- If you are taking medication and you’re still not feeling well, your doctor can change the dose to help manage the condition better
- You should have a blood test at least once a year if you have already been diagnosed with a thyroid problem
- You will need a TSH test once a year if you have an underactive thyroid
- If you fall pregnant and have a thyroid disorder you should have a blood test during early pregnancy
- You shouldn’t alter your medication dose without consulting a doctor
Informed Health Online [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-. How does the thyroid gland work? 2010 Nov 17 [Updated 2018 Apr 19]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279388/
Lab Tests Online UK. (2019). Thyroid Function Tests. Available at: https://labtestsonline.org.uk/tests/thyroid-function-tests
Society for Endocrinology. (2018). Triiodothyronine. Available at: http://www.yourhormones.info/hormones/triiodothyronine/
Society for Endocrinology. (2018). Thyroxine. Available at: http://www.yourhormones.info/hormones/thyroxine/