Abnormal Thyroid Levels Explained
A plain-English guide to TSH, T3, and T4 results — what high and low levels mean, common thyroid patterns, and when your GP will refer you to a specialist.
Quick Guide to TSH, T3, and T4
Your thyroid is a butterfly-shaped gland in your neck that controls metabolism, energy, heart rate, body temperature, and mood. It is regulated by a feedback loop with the pituitary gland in your brain. When thyroid hormones are low, the pituitary releases more TSH to tell the thyroid to work harder — and vice versa.
TSH
Produced by the pituitary gland to tell the thyroid how much hormone to make. TSH is the primary screening test. High TSH means your thyroid is underperforming (hypothyroid). Low TSH means your thyroid is overperforming (hyperthyroid). Think of it as an inverse signal — when the thyroid is sluggish, the pituitary shouts louder (high TSH).
Free T4
The main hormone your thyroid produces. T4 is a storage hormone that gets converted to the active form (T3) in your tissues. Free T4 measures the unbound, biologically available portion. Low fT4 with high TSH confirms hypothyroidism. High fT4 with low TSH confirms hyperthyroidism.
Free T3
The active thyroid hormone. T3 is 3–5 times more potent than T4 at the cellular level. Not routinely tested unless hyperthyroidism is suspected or T4 levels do not match symptoms. Some patients have T3 thyrotoxicosis where T3 is high but T4 is normal.
Thyroid Antibodies
Thyroid peroxidase antibodies (TPO-Ab) are elevated in approximately 95% of Hashimoto thyroiditis cases. Thyroglobulin antibodies (Tg-Ab) are elevated in approximately 60%. TSH receptor antibodies (TRAb) are elevated in Graves disease. These help identify the cause of thyroid dysfunction, not just the presence of it.
Common Thyroid Result Patterns
Thyroid diagnosis depends on the pattern of results, not a single number. Here are the most common combinations your GP looks for.
Overt Hypothyroidism
Thyroid gland is failing. Most commonly Hashimoto thyroiditis (autoimmune). Symptoms: fatigue, weight gain, cold intolerance, dry skin, constipation, brain fog.
Action: Thyroxine (Levothyroxine) replacement. PBS-listed. Lifelong treatment.
Subclinical Hypothyroidism
The thyroid is struggling but still producing enough T4. Very common — affects approximately 5–10% of Australian women. May or may not cause symptoms. Can progress to overt hypothyroidism (2–5% per year).
Action: Retest in 3–6 months. Treat if TSH >10, symptoms present, or thyroid antibodies positive.
Overt Hyperthyroidism
Thyroid is overproducing hormones. Usually Graves disease (autoimmune) or toxic nodular goitre. Symptoms: anxiety, tremor, weight loss, heat intolerance, rapid heart rate, insomnia.
Action: Urgent GP review. Anti-thyroid medication (carbimazole), radioiodine, or surgery.
Subclinical Hyperthyroidism
TSH is suppressed but T4/T3 are still in range. Associated with increased risk of atrial fibrillation and osteoporosis in older adults. May resolve spontaneously.
Action: Retest in 6–12 weeks. Investigate cause. Monitor bone density in postmenopausal women.
T3 Thyrotoxicosis
Rare variant of hyperthyroidism where only T3 is elevated. More common in toxic nodular goitre than Graves disease. Easy to miss if T3 is not tested.
Action: Same as overt hyperthyroidism. Anti-thyroid medication.
Sick Euthyroid (Non-Thyroidal Illness)
Not a thyroid problem. Seen in patients who are acutely unwell (hospitalised, post-surgery, severe illness). The body deliberately suppresses thyroid function to conserve energy during illness.
Action: No thyroid treatment. Retest 6–8 weeks after recovery. Thyroid function normalises.
High TSH — Hypothyroidism
A high TSH is the hallmark of an underactive thyroid. The pituitary gland is releasing more TSH because the thyroid is not producing enough hormones. Hypothyroidism is the most common thyroid disorder in Australia, affecting up to 1 in 20 women and 1 in 100 men.
The most common cause is Hashimoto thyroiditis — an autoimmune condition where the immune system gradually destroys the thyroid gland. Other causes include previous thyroid surgery, radioiodine treatment, medications (lithium, amiodarone), and iodine deficiency (rare in Australia due to iodised salt).
Symptoms develop gradually and are often attributed to ageing or stress: fatigue, weight gain, feeling cold, constipation, dry skin, thinning hair, low mood, poor concentration, and heavy periods. Many patients report feeling not themselves for months before being diagnosed.
Low TSH — Hyperthyroidism
A low or suppressed TSH indicates an overactive thyroid. The pituitary has stopped sending TSH because the thyroid is already producing too much hormone. Hyperthyroidism is less common than hypothyroidism but can be more acutely dangerous due to cardiac effects.
The most common cause is Graves disease — an autoimmune condition where antibodies stimulate the thyroid to overproduce hormones. It occurs more often in younger women (20–40 years) and can be triggered by stress, pregnancy, or smoking.
Hashimoto Thyroiditis vs Graves Disease
Both are autoimmune thyroid conditions, but they cause opposite problems. Understanding which one you have determines your treatment plan.
What it is
Autoimmune destruction of the thyroid gland — the immune system gradually destroys thyroid tissue
Autoimmune stimulation of the thyroid — antibodies mimic TSH and overstimulate the gland
Thyroid function
Hypothyroid (underactive) — too little hormone
Hyperthyroid (overactive) — too much hormone
Key antibody
TPO antibodies (Anti-TPO) — positive in approximately 95%
TSH receptor antibodies (TRAb) — positive in approximately 98%
Typical TSH pattern
High TSH, low/normal T4
Very low TSH, high T4/T3
Onset
Gradual — develops over months to years. Often diagnosed after routine blood test
Can be sudden — weeks to months. Often triggered by stress, pregnancy, or infection
Prevalence in Australia
Most common cause of hypothyroidism. Approximately 5% of women, 1% of men
Most common cause of hyperthyroidism. Approximately 0.5% of women, 0.1% of men
Treatment
Levothyroxine (T4 replacement) — daily tablet, PBS-listed, lifelong
Carbimazole (anti-thyroid) initially, then radioiodine or surgery. May need T4 replacement after treatment
Other features
Often associated with other autoimmune conditions (coeliac disease, Type 1 diabetes, vitiligo)
Can cause Graves ophthalmopathy (bulging eyes) in 25–50% of patients
Medications That Affect Thyroid Results
Several common medications can alter thyroid test results. Always tell your GP about everything you are taking, including supplements.
Levothyroxine (Oroxine, Eutroxsig)
The standard treatment for hypothyroidism in Australia. Dose adjustments are based on TSH levels. Target TSH is usually 0.5–2.5 mU/L. Recheck TSH 6–8 weeks after any dose change. Take on an empty stomach, 30–60 minutes before breakfast. Calcium, iron, and PPIs reduce absorption.
Carbimazole (Neo-Mercazole)
First-line anti-thyroid medication for Graves disease in Australia. Blocks thyroid hormone synthesis. Monitor fT4 and TSH every 4–6 weeks initially. Watch for sore throat and fever — rare but serious side effect (agranulocytosis) requires urgent FBC.
Lithium
Used for bipolar disorder. Inhibits thyroid hormone release and causes hypothyroidism in 20–30% of patients. Monitor TSH every 6 months while on lithium. Levothyroxine may be needed concurrently.
Amiodarone
Anti-arrhythmic medication. Contains high levels of iodine. Can cause both hypothyroidism (more common) and hyperthyroidism (more dangerous). Thyroid function should be checked before starting and every 6 months during treatment.
Biotin supplements
High-dose biotin (over 5mg per day, common in hair and nail supplements) interferes with thyroid assays. Can falsely lower TSH and falsely raise fT4, mimicking Graves disease on blood tests. Stop biotin 48–72 hours before thyroid testing.
When to See an Endocrinologist
Most thyroid conditions are managed by GPs in Australia. However, your GP will refer you to an endocrinologist if:
Hyperthyroidism (Graves disease or toxic nodular goitre) is suspected or confirmed
Thyroid nodules are found on examination or ultrasound
TSH remains abnormal despite appropriate levothyroxine dosing
You are pregnant with thyroid disease or planning pregnancy
Thyroid cancer is suspected (hard nodule, rapid growth, voice changes)
Amiodarone-induced thyroid dysfunction
Children or adolescents with thyroid disease
Complex cases with multiple autoimmune conditions
Frequently Asked Questions
My TSH is 5.2 mU/L. Is that abnormal?
It depends on the reference range used by your lab (most use 0.4–4.0 mU/L, some up to 4.5). A TSH of 5.2 is mildly elevated and falls in the subclinical hypothyroidism range. It does not mean you need treatment now, but it does warrant a retest in 3–6 months. If you have symptoms (fatigue, weight gain, feeling cold) or positive thyroid antibodies, your GP may start treatment.
How often should thyroid levels be checked?
If you are on levothyroxine: every 6–8 weeks after a dose change, then every 6–12 months once stable. If you have subclinical hypothyroidism: every 6–12 months. If you have positive thyroid antibodies with normal TSH: annually. For the general population without risk factors, the RACGP does not recommend routine screening, but thyroid tests are often included in comprehensive blood panels.
Can thyroid problems cause anxiety and depression?
Yes, strongly. Hypothyroidism commonly causes depression, brain fog, and fatigue. Hyperthyroidism commonly causes anxiety, irritability, insomnia, and panic-like symptoms. Many patients are treated for anxiety or depression for years before a thyroid test reveals the underlying cause. If you have been started on antidepressants or anti-anxiety medication without ever having thyroid function checked, ask your GP.
Does pregnancy affect thyroid results?
Yes. In the first trimester, hCG (pregnancy hormone) stimulates the thyroid, which naturally lowers TSH. TSH below 0.4 in the first trimester is often normal. Different reference ranges apply in pregnancy: first trimester TSH 0.1–2.5, second trimester 0.2–3.0, third trimester 0.3–3.0. Untreated hypothyroidism in pregnancy increases risk of miscarriage, pre-eclampsia, and developmental delay. All pregnant women with a history of thyroid disease should be monitored each trimester.
Is thyroid testing covered by Medicare?
Yes. TSH, fT4, and fT3 are bulk-billed when ordered by a GP with a valid clinical indication. Thyroid antibody tests (TPO-Ab, TRAb) are also covered when investigating the cause of abnormal thyroid function. There is no out-of-pocket cost at bulk-billing pathology labs. Your GP does need to justify the test on the request form — routine screening without risk factors is not recommended.
Related Reading
Track Your Thyroid Levels Over Time
Upload your blood test results and see TSH, T3, and T4 trends charted automatically — making it easy to see whether your thyroid medication is working and your levels are stabilising.
This information is based on guidelines from the Royal College of Pathologists of Australasia (RCPA), the Australian Thyroid Foundation, and the RACGP. Reference ranges may vary between pathology providers. SmarterBlood provides educational information only and is not a substitute for professional medical advice.