A woman I work with (I'll call her Sarah) spent the better part of two years being told she was stressed.

She was tired. She was gaining weight despite eating the same way she always had. She was cold all the time, even in summer. Her hair was thinning. Her skin was dry. She couldn't concentrate at work. She felt flat. Not depressed, exactly, but like someone had turned down the brightness on everything by about 30%.

She went to her GP twice. Both times, the advice was essentially: slow down, sleep more, maybe try meditation. Nobody checked her thyroid.

When she finally pushed for blood work, her TSH came back at 14. Her words were "I told them something was wrong and I wasn't leaving without a blood test." Her Free T4 was below range. Textbook hypothyroidism. She started levothyroxine within the week and said the difference, over the next few months, was like coming out of a fog she hadn't realised she was standing in.

I tell this story not because every tired person has a thyroid problem. Most don't. But Sarah's experience highlights something that frustrates me about how thyroid disease gets missed. The symptoms are so generic, so easily attributed to lifestyle or stress or "just getting older," that it can take years to get a diagnosis. When the fix is a daily tablet that costs almost nothing, that delay feels unnecessary.

Thyroid disorders affect an estimated 1 in 33 Australians, with hypothyroidism being the most common. The Australian Thyroid Foundation estimates over one million Australians are living with an undiagnosed thyroid condition. Women are affected roughly ten times more often than men, and the prevalence increases with age. Studies in older Australians suggest around 14% have a clinically relevant thyroid disorder.

This article covers what thyroid function tests actually measure, what the results mean, and when testing is worthwhile. If you've been feeling off and nobody can explain why, this might be one of the things worth checking.

A note before we get into it

What your thyroid does (and why it matters more than you'd think)

Your thyroid is a small, butterfly-shaped gland at the front of your neck. It's easy to forget it's there — until it stops working properly.

The simplest way to think about it: your thyroid works like a thermostat for your metabolism, setting the pace for how quickly your body burns energy. When it's calibrated correctly, you don't notice it. When it's off, either too high or too low, it affects virtually everything.

The thyroid produces two main hormones: thyroxine (T4) and triiodothyronine (T3). T4 is the storage form. Your thyroid makes a lot of it, and your body converts it to T3 as needed. T3 is the active form — it's what actually drives metabolic activity in your cells.

The whole system is regulated by the pituitary gland, which produces thyroid-stimulating hormone (TSH). TSH tells the thyroid how much T4 and T3 to make. When thyroid hormone levels are low, the pituitary ramps up TSH to push the thyroid harder. When levels are high, TSH drops.

This feedback loop is why TSH is such a useful screening tool. It's the first number to move when something goes wrong — often before T4 or T3 levels have drifted outside the reference range. An elevated TSH is the pituitary shouting at the thyroid to work harder. A suppressed TSH means the pituitary is backing off because there's already too much hormone circulating.

The tests: what TSH, Free T4, and Free T3 actually tell you

Hypothyroidism: the slow fade that looks like everything else

Hypothyroidism (an underactive thyroid) is the most common thyroid disorder in Australia, affecting approximately 1 in 33 people. The most common cause is Hashimoto's disease.

The challenge with hypothyroidism is that it develops gradually. The symptoms creep in over months or years, and each one individually could be explained by something else:

Fatigue and low energy. Weight gain (typically modest, 3 to 5 kilograms). Sensitivity to cold. Constipation. Dry skin and hair. Hair thinning or loss. Muscle weakness and aches. Difficulty concentrating — "brain fog." Low mood or depressive symptoms. Menstrual irregularities (heavier or more frequent periods). Slowed heart rate. Puffy face.

Sarah had almost all of these. But individually, each symptom had a plausible alternative explanation: stress, ageing, seasonal changes, not enough sleep. It was only when someone looked at the thyroid that the pattern clicked.

Hypothyroidism affects ten times more women than men, and the symptoms overlap heavily with the things women are most likely to be told are "just stress." That overlap isn't just clinically challenging. It's a structural problem in how these symptoms get investigated.

Hyperthyroidism: the other end of the spectrum

Hyperthyroidism (an overactive thyroid) is less common than hypothyroidism but can be more clinically urgent. The most common cause is Graves' disease, another autoimmune condition, where antibodies stimulate the thyroid to overproduce hormones.

Symptoms tend to be the opposite of hypothyroidism: unexplained weight loss, rapid or irregular heartbeat, anxiety and irritability, tremor, heat intolerance, sweating, difficulty sleeping, frequent bowel movements, and sometimes visible swelling of the thyroid (goitre) or changes in the eyes (especially in Graves' disease).

Hyperthyroidism requires prompt assessment and management, mainly because of its cardiovascular effects. If your TSH comes back very low or suppressed, your GP will typically investigate further and may refer you to an endocrinologist.

Subclinical thyroid dysfunction: the grey zone

Subclinical hypothyroidism is one of the most common and most debated areas in thyroid medicine. It's defined as a mildly elevated TSH (typically 4–10 mU/L) with a normal Free T4. The thyroid is starting to struggle, but it's still producing enough hormone. For now.

The prevalence of subclinical hypothyroidism in Australia is estimated at 4–5%, higher in women and in older adults. In people over 80, it may affect up to 15%.

Here's where it gets complicated. Some people with subclinical hypothyroidism feel genuinely symptomatic (tired, foggy, gaining weight) and improve with treatment. Others feel fine and may never progress to overt disease. Recent evidence, especially in older adults, suggests that treating subclinical hypothyroidism with thyroid hormone replacement doesn't always improve symptoms or outcomes. It may even cause harm through overtreatment.

The current Australian approach, broadly, is:

TSH above 10 mU/L with symptoms: treatment usually recommended. TSH 4–10 mU/L with symptoms: treatment may be considered, especially in younger patients, those with positive antibodies, or women planning pregnancy. TSH mildly elevated in older adults without symptoms: watch and monitor, not treat.

This is a genuine clinical grey zone, and reasonable clinicians can disagree on management. If you're in this space, the most productive approach is to work with your GP, understand the evidence, and make a shared decision based on your specific situation.

Who should be testing

Thyroid testing isn't recommended as a routine population screen. But it's appropriate, and arguably underutilised, in specific situations.

Women with fatigue, weight changes, or mood symptoms that don't have another explanation. Given the prevalence and the sex distribution, thyroid function should be on the differential for any woman presenting with these concerns.

Women planning pregnancy or in early pregnancy. Even subclinical thyroid dysfunction can affect fertility, miscarriage risk, and foetal brain development. Many clinicians recommend TSH screening in early pregnancy or when planning conception.

Anyone with a family history of thyroid disease or autoimmune conditions. Hashimoto's has a strong genetic component.

People with positive thyroid antibodies. Annual TSH monitoring is recommended.

People with symptoms consistent with hypothyroidism or hyperthyroidism. The symptom list is broad, but if multiple thyroid-related symptoms are clustering, get tested.

People with other autoimmune conditions. Type 1 diabetes, coeliac disease, rheumatoid arthritis, and other autoimmune conditions are associated with increased thyroid disease risk.

People on medications that affect thyroid function. Lithium, amiodarone, certain immunotherapies, and some other medications can impact the thyroid.

Older adults with new cognitive changes, fatigue, or unexplained weight changes. Thyroid dysfunction in older adults can present atypically and is easily mistaken for age-related decline.

How to get useful results

Understanding your numbers

Here's a general framework. These ranges are approximate, and your lab may use slightly different values.

Marker	Range	What it suggests
TSH	0.4–4.0 mU/L	Generally normal
TSH	Above 4.0 mU/L	Elevated. Suggests possible hypothyroidism. If persistent, warrants Free T4 check.
TSH	Above 10 mU/L	Very elevated. Overt hypothyroidism likely if Free T4 is low.
TSH	Below 0.4 mU/L	Suppressed. Suggests possible hyperthyroidism. Warrants Free T4 and Free T3.
Free T4	~10–20 pmol/L	Normal
Free T4	Below range + elevated TSH	Overt hypothyroidism
Free T4	Within range + elevated TSH	Subclinical hypothyroidism
Free T4	Above range + suppressed TSH	Hyperthyroidism
Free T3	~3.5–6.5 pmol/L	Normal. Most useful when hyperthyroidism is suspected, or when treated and still symptomatic.

These are guidelines, not verdicts. One abnormal result doesn't equal a diagnosis. Confirmation with repeat testing, correlation with symptoms, and clinical context are all part of the picture.

What happens after an abnormal result

Elevated TSH with low Free T4 (overt hypothyroidism):

Your GP will likely start you on levothyroxine — a synthetic form of T4 that replaces what your thyroid isn't producing. The dose is usually started conservatively and adjusted over 6–8 weeks based on repeat TSH testing. Most people find the right dose within a few adjustments.

Levothyroxine is taken daily, usually first thing in the morning on an empty stomach, at least 30 minutes before food. It's generally well-tolerated. You're replacing a hormone your body should be making, not adding something foreign.

Elevated TSH with normal Free T4 (subclinical hypothyroidism):

As discussed above, this is a clinical judgement call. Your GP may recommend monitoring (repeat TSH in 6–12 weeks), treatment (especially if you're symptomatic, young, or planning pregnancy), or referral to an endocrinologist for complex cases.

Suppressed TSH with elevated Free T4 or Free T3 (hyperthyroidism):

Further investigation is needed — typically thyroid antibodies (TSH receptor antibodies for Graves' disease), possibly a thyroid scan, and potentially referral to an endocrinologist. Treatment depends on the cause but may include antithyroid medication, radioactive iodine, or surgery.

Positive thyroid antibodies with normal function:

No treatment needed, but annual TSH monitoring is recommended. Knowing you have positive antibodies gives you and your clinician a heads-up that your thyroid is under immune surveillance and may need support down the line.

Tests to consider through Bloody Good

The core thyroid test:

Thyroid Function Test (TFT) covers TSH, Free T4, and Free T3. That's the full functional picture.

Related tests worth considering alongside thyroid:

Iron Studies (Including Ferritin). Iron deficiency causes identical fatigue symptoms and commonly coexists with thyroid dysfunction.

Vitamin D (25-OH). Often low alongside thyroid issues.

Vitamin B12. B12 deficiency is more common in people with autoimmune thyroid disease.

Full Blood Count (FBC). Anaemia is a common companion to hypothyroidism.

Cholesterol (Lipid Studies inc. HDL). Hypothyroidism can elevate cholesterol levels. Research suggests hypothyroid patients have total cholesterol roughly 0.36 mmol/L higher than euthyroid individuals.

HbA1c. Metabolic health context.

If you want a full baseline:

The Bloody Good Test covers 100 biomarkers including full thyroid function alongside iron, cholesterol, liver, kidney, vitamin D, blood sugar, and more. If thyroid is one concern among several, the broader option makes sense.

Biomarker info: TSH

When to retest

After starting levothyroxine: Retest TSH (and Free T4) at 6–8 weeks. Dose adjustments are based on this follow-up result. Repeat every 6–8 weeks until TSH is stable in the target range.

Once stable on medication: Annual TSH monitoring is standard. More frequent if symptoms change, dose is adjusted, or there's a major life event (pregnancy, significant weight change, new medication).

If subclinical and not treating: Repeat TSH in 6–12 weeks initially, then every 6–12 months if it remains mildly elevated but stable.

If antibodies are positive but function is normal: Annual TSH check to catch any progression early.

If you've had a normal result but develop symptoms later: New-onset fatigue, weight changes, cold intolerance, hair loss, or menstrual changes warrant retesting — even if a previous thyroid check was normal. Thyroid disease can develop at any age.

During pregnancy or when planning conception: TSH should be checked in early pregnancy (or ideally before conception) and monitored through pregnancy if there's a history of thyroid disease or positive antibodies. Target TSH ranges in pregnancy are trimester-specific and tighter than the general population.

Explore more biomarkers

If you want to go deeper into any of the markers covered here, the Bloody Good biomarker directory has detailed pages on what each test measures and how to think about results.

Browse the Bloody Good Biomarker Directory