Thyroid Function Testing: TSH, fT4 and fT3 Explained

Thyroid function plays a vital role in your overall health, regulating energy levels, metabolism, mood and body temperature. When the thyroid gland is not working optimally, it can lead to conditions such as hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid). Understanding your thyroid status often involves measuring key hormones in the blood, including thyroid‑stimulating hormone (TSH), free thyroxine (fT4) and free tri‑iodothyronine (fT3).

Below, we explain what these tests measure, why they matter, and how clinical guidelines from the National Institute for Health and Care Excellence (NICE) and the NHS guide appropriate testing. Our purpose is to help you understand your options and make informed decisions, not to diagnose or replace clinical assessment by a healthcare professional.

How Thyroid Function Tests Work

Thyroid‑stimulating hormone (TSH) is typically the first marker assessed when thyroid dysfunction is suspected. It is produced by the pituitary gland and signals the thyroid how much hormone to produce. If TSH is outside the normal range, it may suggest the thyroid is underactive or overactive.

If TSH results fall outside the reference range, additional hormones such as fT4 and fT3 are often measured:

  • fT4 (free thyroxine): the main hormone produced by the thyroid.

  • fT3 (free tri‑iodothyronine): the active form of thyroid hormone, which influences metabolic activity. (NICE)

This cascading approach: starting with TSH and adding fT4 and/or fT3 as needed — reflects clinical best practice and helps avoid unnecessary testing. (NICE)

What the NICE Guidelines Say

The current NICE guideline on thyroid disease (NG145) provides evidence‑based recommendations on when to consider testing for suspected thyroid dysfunction:
NICE: Thyroid disease: assessment and management (NG145) — https://www.nice.org.uk/guidance/ng145/ (NICE)

Key points from the guideline include:

  • TSH should be the first test for adults when secondary thyroid dysfunction is not suspected.

  • If TSH is above the reference range, fT4 should be measured.

  • If TSH is below the reference range, both fT4 and fT3 should be measured.

  • These tests may be repeated if symptoms change or worsen, but not sooner than about six weeks after the previous test. (NICE)

The guideline also notes that symptoms of thyroid dysfunction may sometimes be mistaken for other conditions (e.g., menopausal symptoms), and testing should be targeted appropriately rather than done routinely without clinical suspicion. (NICE)

How the NHS Uses Thyroid Testing

GPs commonly use TSH as the frontline test for suspected thyroid dysfunction. Based on the result, the laboratory may automatically include fT4 and/or fT3 as per clinical pathways. (pathology.uhsussex.nhs.uk)

The NHS also provides general guidance that thyroid function tests are not routinely conducted during acute illness unless there is a clear clinical reason, and testing solely because of unrelated conditions (for example type 2 diabetes without symptoms of thyroid dysfunction) is not generally recommended. (NICE)

Why TSH, fT4 & fT3 Matter

  • TSH indicates how hard the pituitary gland is trying to stimulate the thyroid.

  • fT4 reflects how much thyroid hormone is available in the bloodstream.

  • fT3 is a more active form and can help clarify thyroid hormone action, though it is not always measured unless clinically indicated. (NICE)

Together, these tests help narrow down whether the thyroid is underactive, overactive, or functioning within expected ranges.

What This Test Can (and Can’t) Tell You

Can help you and your healthcare professional:

  • Understand whether your thyroid function is within typical ranges

  • Explore whether symptoms might be associated with thyroid hormone levels

  • Provide data to discuss with a GP or endocrinologist

Cannot:

  • Diagnose conditions on its own

  • Replace clinical assessment or judge treatment needs

  • Predict outcomes without clinical context

Next Steps After Testing

If your results fall outside the reference ranges, we recommend discussing them with a clinician. They can interpret results in light of your symptoms, medical history, and any medications you are taking.

Many clinicians may start with TSH and add additional markers as needed, following NHS and NICE pathways to guide appropriate investigation. (NICE)

Test NICE Recommendation NHS Guidance Recommended Frequency Notes Product Link
TSH (Thyroid Stimulating Hormone) First-line test for diagnosing hypothyroidism or hyperthyroidism NHS recommends TSH as the primary test if symptoms suggest thyroid disorder Baseline if symptomatic; repeat 6–12 weeks after starting or adjusting treatment Key marker of thyroid function; guides medication dosing Link to Thyroid Profile Test
Free T4 (Thyroxine) Measured if TSH is abnormal to confirm thyroid status NHS recommends Free T4 alongside TSH for diagnosis and monitoring At diagnosis and after treatment adjustments Determines degree of thyroid hormone deficiency/excess Link
Free T3 (Triiodothyronine) Used if TSH abnormal or suspected hyperthyroidism NHS notes T3 may be measured in hyperthyroid cases As clinically indicated More sensitive for hyperthyroidism; often combined with TSH/Free T4 Link
Anti-TPO / Thyroid Antibodies (optional) Not routine; used if autoimmune thyroid disease suspected NHS advises antibodies if autoimmune thyroiditis suspected or unexplained abnormal TSH One-off at diagnosis if indicated Helps differentiate autoimmune thyroid disorders Not currently offered by LHC

References & Further Reading

Frequently Asked Questions (FAQs)

How is the test performed? 

You will receive a complete blood collection kit by post. The kit contains all you need to collect blood from your finger. The components of the kit are: 

1. Instruction Sheet: Provides detailed steps on how to collect the blood sample correctly. 

2. Box: Contains all the components and is used to hold the tube while collecting blood. 

3. Lab Request Form: Needs to be filled out and included with the sample. 

4. Labels: For labeling the sample tube. 

5. Tube Pouch: A protective pouch for the sample tube. 

6. UN3733 Prepaid Mailing Bag: For sending the sample back to the lab. 

7. Lancets: Used to prick your finger for blood collection. 

8. Wipes: For cleaning your finger before and after collection. 

9. Plasters: To cover your finger after blood collection. 

10. Blood Collection Tube: for collecting the blood sample. 

These components are essential to ensure that the blood sample is collected, stored and sent correctly for accurate testing. 

Do I need to set up an account?

No, you don’t need to set up an account, but you do need to activate the sample ID before sending your blood sample to the lab.

Should I fast before collecting my blood sample? 

Please check individual test pages for details.

I couldn’t collect enough blood, what should I do? 

If you read the instruction sheet carefully and follow the steps as directed, then you should not have a problem in collecting the right amount of blood. However, if you followed all the steps and couldn’t get enough blood, i.e., to the upper line, please contact us and we will arrange to send you a fresh tube and 4 lancets (for an extra fee) and not for free). Please make sure you keep any unused elements (instruction sheet, box, lab request form, labels, tube pouch, and UN3733 prepaid mailing bag) from the kit as you will need them when you receive the new tube and lancets.

I sent my sample, and the blood wasn’t enough, and I didn’t contact London Health Company. What’s going to happen now? 

Don’t send any sample if the blood isn’t enough unless you contactedus and we told you to do so. If you sent the sample and the blood wasn’t enough, the lab will reject your sample and it won’t be processed.

After I closed the yellow tube cap, and inverted it 5-10 times gently, I noticed the waxy substance in the tube didn’t dissolve or move, is this OK? 

Yes, this is fine. When inverting the tube, the blood will mix with the waxy substance, but you won’t be able to see any difference. The waxy substance will not move or dissolve.

The yellow-top tube has waxy substance, but the purple top tube looks empty. Is it a faulty tube? 

The purple top tube is perfectly fine and should look empty.

Where do I find my sample ID?

Your sample ID is unique and should be on the small lab request slip and is highlighted. Your sample ID should always start with three letters and has no spaces.

What happens if I forgot to activate my sample ID before sending the sample to the lab? 

This will cause significant delays in reporting the sample if it arrives at the lab and is not yet activated.

What if I forgot to include the lab request slip/form with my sample? 

That shouldn’t be a big problem. You can still receive your report on time, but you need to make sure that the sample ID is activated before the sample arrives at the lab. If the lab request slip/form isn’t included and the sample ID isn’t activated, the lab will discard the sample and no refund will be issued.

I bought my test kit online, does that mean that you have my details, and I don’t need to activate my sample ID? 

You still need to activate your sample ID. By activating your sample ID, your blood sample is linked to you. Without activating your sample ID, your blood sample won’t be traced back to you.

Do I need to drop off my blood sample at the post office? 

Yes, please do so. Make sure that it’s dropped off before the last collection time; otherwise, it will stay in the post office overnight and the lab won’t get it the next day.

Why do I need to avoid posting my blood sample on Friday and weekends? 

We aim to receive your blood sample within 24 hours of blood collection. If the sample is posted on a Friday, it means that it will stay in the post for 3 days.

How long does it take for the lab to receive my sample?

Generally, samples arrive within 24 hours of posting to the lab. We use Royal Mail Tracked 24 service, but sometimes, there might be a slight delay in delivery. If the sample’s integrity was compromised by delays, we send a free replacement kit if you have the tracking number to prove the time of posting. This is if sent on Mon, Tue or Wed.

How will I know if my sample was received by the lab? 

You will receive an email notification confirming that your sample was checked into the lab.

How will I receive my results? 

Once your report is checked and ready, it will be sent to your registered email.

Why have I not received all my results? 

This is usually due to one of two reasons: 

1. Late or non-activation of the sample ID. 

2. Entering the wrong sample ID. 

Why does my report say that my sample was rejected by the lab? 

Sample rejection mainly occurs due to haemolysis, which is when the red blood cells in the blood collection tube start to break down. This impacts the lab ability to generate results for some tests. 

 

Haemolysis can happen at any stage of the process but most commonly occurs during blood collection. It often results from squeezing your finger instead of massaging the side of your hand. This only happens in a small number of cases, and we will provide a second kit free of charge. 

 

Another reason for sample rejection is when a person has been unable to collect the necessary amount of blood required to perform the testing and sends it to the lab without contacting us beforehand. 

Can I change my address after I checked out?

Please note that we cannot change your address if it was entered incorrectly, as orders are dispatched automatically. Once labels are printed, it is difficult for our staff to locate and retract them.

Is the return Free?

Returns must be sent at the buyer's expense and must be returned via tracked mail. We do not accept returns for opened kits as this is a medical device.

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