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

NICE guideline NG145: Thyroid disease  assessment and management — https://www.nice.org.uk/guidance/ng145/ (NICE)

NHS Primary NHS overview on thyroid testing practices (local NHS pathology references) (pathology.uhsussex.nhs.uk)