Thyroid Blood Tests: The Complete Guide
Thyroid disease affects 1 in 20 Australians — and women are 5 to 8 times more likely to be diagnosed. A simple blood test can detect problems years before symptoms become severe.
Why Thyroid Testing Matters
Your thyroid gland is a butterfly-shaped organ at the base of your neck that produces hormones controlling your metabolic rate, heart rate, body temperature, weight, energy levels, and mood. When the thyroid malfunctions, the effects are widespread and often mistaken for other conditions — depression, menopause, ageing, or simple fatigue.
In Australia, approximately 850,000 people are living with a diagnosed thyroid condition, but many more remain undiagnosed. Hypothyroidism (underactive thyroid) is far more common than hyperthyroidism (overactive thyroid), with Hashimoto's thyroiditis — an autoimmune condition — being the leading cause. Risk factors include female sex, age over 60, family history of thyroid disease, other autoimmune conditions (type 1 diabetes, coeliac disease), and previous neck radiation.
The good news is that thyroid disease is almost always diagnosable with straightforward blood tests and treatable with affordable medication. TSH testing is bulk billed under Medicare and is one of the most commonly ordered pathology tests in Australian general practice.
The 4 Key Thyroid Tests Explained
TSH (Thyroid Stimulating Hormone)
What it measures: TSH is the single most important thyroid test and the recommended first-line screening test by the RCPA, Endocrine Society of Australia, and virtually all international guidelines. Produced by the pituitary gland, TSH acts as a thermostat for the thyroid — it rises when the thyroid is underactive (hypothyroidism) and falls when the thyroid is overactive (hyperthyroidism). TSH is exquisitely sensitive to small changes in thyroid hormone levels, often becoming abnormal before T4 or T3 move outside their normal range. This makes it the best early detection test for thyroid dysfunction.
When it's ordered: As a first-line screening test when thyroid dysfunction is suspected based on symptoms (fatigue, weight changes, cold/heat intolerance, mood changes). Also monitored regularly in patients on thyroid medication (levothyroxine or carbimazole), every 6–12 months once stable, and during pregnancy in women with known thyroid disease or risk factors.
How to interpret: Normal: 0.4–4.0 mU/L (varies slightly by lab). High TSH (above 4.0 mU/L) suggests hypothyroidism — the pituitary is working harder because the thyroid is underperforming. Low TSH (below 0.4 mU/L) suggests hyperthyroidism — the pituitary is suppressed because the thyroid is overproducing. However, TSH alone does not tell the full story — Free T4 and Free T3 are needed to confirm.
Free T4 (Free Thyroxine)
What it measures: Free T4 is the unbound, biologically active form of thyroxine — the main hormone produced by the thyroid gland. While 99.97% of T4 in the blood is bound to proteins (and inactive), it is the tiny free fraction that enters cells and drives metabolism. Free T4 is always ordered alongside an abnormal TSH to confirm and quantify thyroid dysfunction. It is more stable and reliable than Total T4, which is affected by protein levels that change with pregnancy, oestrogen therapy, liver disease, and certain medications.
When it's ordered: Whenever TSH is abnormal (high or low). As part of initial thyroid investigation. To monitor levothyroxine replacement therapy — the target is usually mid-normal range. During pregnancy if thyroid disease is known or suspected.
How to interpret: Normal: approximately 10–20 pmol/L (lab-specific). Low Free T4 with high TSH confirms primary hypothyroidism. High Free T4 with low TSH confirms hyperthyroidism. Normal Free T4 with abnormal TSH suggests subclinical disease (the pituitary detects a problem before the hormones themselves become overtly abnormal).
Free T3 (Free Triiodothyronine)
What it measures: Free T3 is the most metabolically active thyroid hormone — approximately 3–5 times more potent than T4. Most T3 is produced by conversion of T4 in peripheral tissues (liver, kidneys, muscles), not directly by the thyroid. Free T3 is particularly important in diagnosing hyperthyroidism, where it may be elevated even when Free T4 is normal (a condition called T3 thyrotoxicosis). It is less useful for monitoring hypothyroidism because the body compensates for low T4 by increasing T4-to-T3 conversion, so T3 often remains normal until hypothyroidism is quite advanced.
When it's ordered: When TSH is low (suppressed) but Free T4 is normal — to detect T3 thyrotoxicosis. In patients with suspected or confirmed Graves’ disease to assess severity. When symptoms of hyperthyroidism persist despite normal Free T4. Free T3 is NOT routinely needed for hypothyroidism monitoring and is not bulk billed by Medicare for this purpose.
How to interpret: Normal: approximately 3.5–6.5 pmol/L (lab-specific). Elevated Free T3 with low TSH confirms hyperthyroidism. Isolated T3 elevation with normal T4 (T3 thyrotoxicosis) occurs in early or mild Graves’ disease and toxic nodular goitre. Low Free T3 in severe non-thyroidal illness is common and does not indicate true hypothyroidism.
Thyroid Antibodies (TPO, TgAb, TRAb)
What it measures: Thyroid antibodies detect autoimmune thyroid disease, which is the most common cause of thyroid dysfunction in Australia. Anti-TPO antibodies are positive in approximately 90% of Hashimoto’s thyroiditis (the leading cause of hypothyroidism) and in about 75% of Graves’ disease. TRAb (TSH receptor antibodies) are specific to Graves’ disease — they stimulate the TSH receptor, causing the thyroid to overproduce hormones. TRAb is also used to predict neonatal thyroid disease in pregnant women with Graves’ disease, as these antibodies cross the placenta.
When it's ordered: Anti-TPO: when hypothyroidism is confirmed (to identify Hashimoto’s) or when subclinical hypothyroidism is found (positive antibodies predict progression to overt hypothyroidism at approximately 5% per year). TRAb: to confirm Graves’ disease, to differentiate Graves’ from other causes of hyperthyroidism, and in the third trimester of pregnancy in women with Graves’ disease.
How to interpret: Anti-TPO above 35 IU/mL is positive (varies by lab). Positive TPO with hypothyroidism equals Hashimoto’s thyroiditis. Positive TPO with normal thyroid function means increased risk of future hypothyroidism (monitor annually). Positive TRAb with hyperthyroidism equals Graves’ disease. TRAb levels correlate with disease activity.
Thyroid Test Patterns: What Your Results Mean
Thyroid results are always interpreted as a pattern, not individual numbers. The combination of TSH, Free T4, and Free T3 tells a specific diagnostic story:
| Pattern | TSH | Free T4 | Free T3 | Diagnosis | Action |
|---|---|---|---|---|---|
| Normal | Normal (0.4–4.0) | Normal | Normal | Euthyroid (normal thyroid function) | No treatment needed. Recheck if symptomatic. |
| Primary hypothyroidism | High (above 10) | Low | Low or normal | Overt hypothyroidism (underactive) | Levothyroxine replacement. Check TPO antibodies. |
| Subclinical hypothyroidism | Mildly high (4–10) | Normal | Normal | Subclinical hypothyroidism | Check TPO antibodies. Treat if TSH >10 or symptomatic. Monitor every 6–12 months. |
| Primary hyperthyroidism | Suppressed (below 0.1) | High | High | Overt hyperthyroidism (overactive) | Urgent referral. TRAb to confirm Graves’. Carbimazole or propylthiouracil. |
| Subclinical hyperthyroidism | Low (0.1–0.4) | Normal | Normal | Subclinical hyperthyroidism | Monitor 6–12 weekly. Treat if persistent, symptomatic, or over 65 (AF risk). |
| T3 thyrotoxicosis | Suppressed | Normal | High | T3 thyrotoxicosis (early Graves’ or toxic nodule) | Check TRAb. Thyroid ultrasound. Endocrine referral. |
| Central hypothyroidism | Low or normal | Low | Low | Secondary hypothyroidism (pituitary problem) | Pituitary MRI. Endocrine referral. Check other pituitary hormones. |
Thyroid Testing in Pregnancy
Thyroid health during pregnancy is critical because the developing baby relies entirely on the mother's thyroid hormones for brain development during the first trimester (before the fetal thyroid starts functioning at 12–14 weeks). Untreated hypothyroidism in pregnancy increases the risk of miscarriage, preeclampsia, premature birth, and impaired neurodevelopment. TSH targets are stricter in pregnancy than in the general population:
First trimester (weeks 1–12) — TSH target: 0.1–2.5 mU/L
hCG naturally suppresses TSH in early pregnancy. TSH below 0.1 with mild symptoms is usually normal. Overt hypothyroidism must be treated promptly as it affects fetal brain development.
Second trimester (weeks 13–26) — TSH target: 0.2–3.0 mU/L
TSH rises slightly as hCG declines. Continue monitoring every 4–6 weeks if on levothyroxine. Dose increase of 25–30% is often needed by week 16–20.
Third trimester (weeks 27–40) — TSH target: 0.3–3.5 mU/L
TSH targets relax slightly. Check TRAb in Graves’ patients (antibodies cross the placenta). Post-partum thyroiditis occurs in 5–10% of women.
Medication Monitoring: Testing on Treatment
If you are taking thyroid medication, regular blood tests are essential to ensure the dose is correct. Both under-treatment and over-treatment carry risks.
Levothyroxine (for hypothyroidism)
Test TSH 6–8 weeks after starting or any dose change
Take blood BEFORE your morning levothyroxine dose
Target TSH: usually 0.5–2.5 mU/L (varies by age and condition)
Once stable, test every 6–12 months
Take levothyroxine on an empty stomach, 30–60 minutes before food
Calcium, iron, and PPI medications interfere with absorption — separate by 4 hours
Carbimazole / PTU (for hyperthyroidism)
Test Free T4 and Free T3 every 4–6 weeks initially
TSH may remain suppressed for months after starting treatment (lag effect)
Target: normalisation of Free T4/T3, then TSH normalisation follows
Monitor FBC for rare but serious side effect: agranulocytosis
Seek urgent medical attention for sore throat or fever while on carbimazole
Treatment duration usually 12–18 months, then trial withdrawal
What to Ask Your GP
Script for your GP appointment:
“I've been experiencing fatigue / weight gain / cold intolerance / anxiety / tremor / hair loss and I have family history of thyroid disease / other autoimmune conditions. Could we check my TSH? If it's abnormal, I'd like Free T4 as well, and thyroid antibodies if hypothyroidism is confirmed.”
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Reference ranges sourced from the Royal College of Pathologists of Australasia (RCPA) and the Endocrine Society of Australia. SmarterBlood provides health information and AI-powered blood test analysis. It is not a substitute for professional medical advice, diagnosis, or treatment.