A thyroid test rarely means just one number. Depending on what's ordered, it can measure TSH, free T3, free T4, thyroid antibodies and reverse T3 - five different markers that each tell a different part of the story. TSH is usually the starting point, but a "normal" TSH doesn't rule out every thyroid-related pattern, which is why people often end up looking into the other four.
This guide walks through what each marker actually shows, how the panels differ, and where to look next if your results come back unremarkable but something still feels off.
What does a thyroid test measure?
Your thyroid is a small gland at the front of the neck that produces hormones controlling metabolism - the rate at which your body uses energy. A thyroid test checks how well that system is working by measuring one or more of the following:
- TSH (thyroid-stimulating hormone) - produced by the pituitary gland to tell the thyroid how much hormone to make
- Free T3 and free T4 - the two hormones the thyroid itself produces
- Thyroid antibodies (TPOAb and TGAb) - markers relevant to autoimmune thyroid conditions
- Reverse T3 - an inactive form of T4 that can affect how much active hormone the body has available to use
Most people start with the first group and only add antibodies or reverse T3 if symptoms persist despite a normal result.
TSH: the first marker, and why "normal" isn't always the end of the story
TSH is the most commonly ordered thyroid marker, and for most people it's a reliable first screen.[3][4] When TSH is high, it usually means the pituitary is working harder to prompt a sluggish thyroid - a pattern linked to hypothyroidism. When TSH is low, the opposite pattern applies, linked to an overactive thyroid.
Where it gets more complicated is when TSH sits within the reference range but symptoms - fatigue, weight change, low mood, hair thinning, feeling the cold more than usual - don't resolve. TSH alone doesn't show free T3 and free T4 levels, thyroid antibodies, or how much of the hormone your body is actually converting to its active form. That's the gap a fuller panel is designed to close.[3][4][8][9][10]
Free T3 and free T4: the hormones that actually drive metabolism
Free T4 is the hormone your thyroid produces in the largest amount. Much of it is then converted into free T3, the active form that has the most direct effect on how your cells use energy.
Looking at free T3 and free T4 alongside TSH gives a fuller picture than TSH on its own - it shows not just whether the pituitary is sending a stronger or weaker signal, but what the thyroid is actually producing in response. This combination is the core of a standard thyroid function test and the usual next step when a TSH-only result doesn't explain how someone feels.
Thyroid antibodies: TPOAb, TGAb and TRAb
Many thyroid conditions are autoimmune. Hashimoto's thyroiditis (linked to an underactive thyroid) and Graves' disease (linked to an overactive thyroid) both involve the immune system producing antibodies against the thyroid gland - but the antibodies involved differ between the two conditions.[2][11][12]
TPOAb (thyroid peroxidase antibodies) and TGAb (thyroglobulin antibodies) are the antibodies associated with Hashimoto's thyroiditis, and testing for them is relevant to identifying whether an autoimmune process is behind an underactive thyroid.[11] Graves' disease is identified by a different antibody, TRAb (TSH-receptor antibody), which isn't part of i-screen's thyroid antibody panels.[1][3][11] If Graves' disease is suspected, TRAb is the specific marker your GP would look at.
This matters because autoimmune thyroid conditions are often monitored differently to non-autoimmune ones. Antibody testing doesn't diagnose Hashimoto's on its own; results are something to discuss with your GP alongside TSH, free T3 and free T4.
Reverse T3: not part of routine testing
Reverse T3 is an inactive form of T4. Under normal conditions, the body converts a portion of T4 into active T3, and a smaller portion into reverse T3, as part of clearing out unneeded hormone.[5] Reverse T3 tends to rise mainly during acute or chronic illness, a pattern sometimes called non-thyroidal illness syndrome, rather than as a marker of an underlying thyroid disorder in someone who is otherwise well.[4][6]
Reverse T3 isn't part of a standard thyroid panel, and current thyroid guidelines don't support using it to guide general symptom work-up or thyroid hormone treatment decisions.[4][5] It's reserved for specific situations - such as suspected non-thyroidal illness, or rarer genetic conditions affecting hormone conversion - rather than an automatic next step once the core markers come back normal. Some emerging research has looked at reverse T3 in people with persistent fatigue on thyroid hormone replacement, but this remains an active area of study rather than established practice, so it's worth discussing with your GP what reverse T3 testing can and can't tell you before ordering it.[7]
Thyroid function test, advanced or comprehensive: how the panels differ
Because the five markers above build on each other, i-screen's thyroid panels are structured in layers rather than as one-size-fits-all. The table below shows what each level includes, so you can see where TSH-only testing stops and where antibodies or reverse T3 come in.
| Panel | Markers included | Best suited to | | --- | --- | --- | | Thyroid Function Test | TSH, free T3, free T4 | A first check, or routine monitoring of a known thyroid condition | | Advanced Thyroid Test | TSH, free T3, free T4, TPOAb, TGAb | Investigating whether an autoimmune process such as Hashimoto's thyroiditis may be involved | | Comprehensive Thyroid Test | TSH, free T3, free T4, TPOAb, TGAb, reverse T3 | Specific situations where a GP also wants to look at reverse T3, such as suspected non-thyroidal illness - not a routine addition to general symptom-checking |
The Advanced and Comprehensive panels test TPOAb and TGAb, the antibodies linked to Hashimoto's thyroiditis. They don't include TRAb, the antibody used to identify Graves' disease.[1][3][11]
All three are blood tests processed by a NATA-accredited Australian laboratory, ordered online with no GP referral needed, and viewed in your i-screen dashboard once results are ready.
When symptoms don't match a "normal" thyroid result
A thyroid panel - at any level - is one piece of a bigger picture, not the whole picture. Fatigue, mood changes, weight fluctuation and poor cold tolerance can all overlap with thyroid patterns, but they also overlap with several other common and checkable causes:
- Iron deficiency is Australia's most common nutritional problem and produces symptoms - tiredness, low energy, poor concentration - that closely mirror an underactive thyroid.[13][14][15]
- B12 and folate deficiency can also drive fatigue and low mood, independently of thyroid function.
- Sex hormones - oestrogen, progesterone and testosterone - interact with thyroid hormone and can produce a similar symptom picture in both women and men.[16][17][18]
If a thyroid panel comes back within range and symptoms persist, these are usually the next places worth looking, rather than repeating the same thyroid markers.
Where to look next
Because thyroid hormone interacts so closely with iron status, B vitamins and sex hormones, the next step for most people isn't another thyroid test - it's checking the systems thyroid function overlaps with.
If your symptoms track with your cycle, fertility, or a broader hormone picture, i-screen's women's health and fertility testing covers oestrogen, progesterone and the nutrient markers that commonly interact with thyroid symptoms.
If you're more focused on energy, recovery or a male hormone picture, i-screen's men's health testing covers testosterone alongside the same nutrient markers.
And if iron, B12 or vitamin D haven't been checked recently, i-screen's nutrition and micronutrient testing - including the standalone Iron Studies Test and B12 and Folate Test - is often the fastest way to rule a common overlap in or out.
This information is not intended to be a substitute for professional medical advice. Always seek the guidance of your doctor or other qualified health professional with any questions about your health or a medical condition. i-screen provides wellness and educational services only and does not diagnose, treat, cure or prevent any disease or medical condition.
