Understanding your thyroid test results chart

If you’ve ever had a thyroid function test (TFT), you’ve probably stared at a confusing lab report filled with markers like TSH, FT4, and FT3, and wondered what on earth it all means.  But know that you’re not alone. Thyroid testing is pretty common, yet many people don’t fully understand what their results are telling them.

What does the thyroid do?

Your thyroid is a small, butterfly-shaped gland that sits at the front of your neck. It might be small, but it plays a big role in how your body works, controlling your metabolism, weight, body temperature, mood, and even how your heart and brain function.¹

The gland produces two main hormones: T3 (triiodothyronine) and T4 (thyroxine). These are regulated by TSH (thyroid-stimulating hormone), which is produced by the pituitary gland in your brain. TSH acts like a signal, telling your thyroid how much hormone to make.¹

Why are thyroid tests done?

Your doctor might order thyroid tests if you have symptoms like tiredness, weight changes, depression, anxiety, irregular periods, or sensitivity to heat or cold. These symptoms could suggest either an underactive thyroid (hypothyroidism) or an overactive thyroid (hyperthyroidism).¹

The main biomarkers tested include:²

• TSH: the most sensitive marker for thyroid issues

• Free T4 (FT4): shows the level of available thyroxine

• Free T3 (FT3): sometimes used to evaluate hyperthyroidism

• Thyroid antibodies: used to detect autoimmune thyroid conditions like Hashimoto’s or Graves’ disease

TSH: the key hormone to watch

Your TSH level is the first place doctors look. Here’s a general guide to adult TSH ranges in the UK:³

• Normal: Around 0.4–4.0 mIU/L

• Low TSH (<0.4): May suggest hyperthyroidism

• High TSH (>4.0): May suggest hypothyroidism

If your TSH is high and your FT4 is low, that’s overt hypothyroidism. If TSH is high but FT4 is normal, that’s subclinical hypothyroidism, which is a milder form that might still need monitoring or treatment, especially if TSH goes above 10 mIU/L.

If your TSH is low and FT4 or FT3 is raised, that indicates hyperthyroidism (also called thyrotoxicosis).

TSH can also be temporarily affected by illness, pregnancy, or certain medications, so your GP may ask for a repeat test after a few months to confirm a diagnosis.

TSH reference ranges by age and sex:^2,3

Thyroid problems in pregnancy can lead to complications like miscarriage, preeclampsia, and premature birth, so regular checks are important if you have a thyroid condition or are at risk.

What if your results are abnormal?

If your test results fall outside the reference ranges, don’t panic. Diagnosis isn’t based on numbers alone. Your symptoms, other blood markers, medications, and health history all matter.

Once treatment begins, your TSH should be rechecked every 2–3 months until stable. The aim is to keep TSH within range and help you feel well. 

When to get tested

You might be tested for thyroid function if you:¹

• Feel unusually tired, anxious, or depressed

• Gain or lose weight without explanation

• Have irregular periods or fertility issues

• Have a family history of thyroid disease

• Are pregnant or trying to conceive

• Are over 60 and develop new symptoms

It’s also important to test regularly if you’re on thyroid medication, as your dose may need adjusting over time.

The numan take

Thyroid hormone tests offer valuable insights into how your body is functioning. While the numbers might seem technical, they help guide important decisions about your health and wellbeing. If you’re unsure what your results mean, speak to your clinician. 

References

1. Armstrong M, Asuka E, Fingeret A. Physiology, thyroid function. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2025.

2. National Guideline Centre (UK). Thyroid function tests: Thyroid disease: assessment and management: Evidence review C. London: National Institute for Health and Care Excellence (NICE); 2019.

3. Aubert CE, Floriani C, Bauer DC, da Costa BR, Segna D, Blum MR, et al. Thyroid function tests in the reference range and fracture: Individual participant analysis of prospective cohorts. The journal of clinical endocrinology and metabolism. 2017;102(8): 2719–2728.