Thyroid Function Tests: TSH, T3, and T4 Explained for Australians

Introduction: Your Thyroid's Critical Role

Your thyroid might be small – just a butterfly-shaped gland at the base of your neck – but it has an enormous impact on your health. This tiny gland controls your metabolism, affecting everything from your energy levels and weight to your heart rate and body temperature.

Thyroid disorders are remarkably common in Australia. Approximately 1 in 20 Australians has some form of thyroid dysfunction, with women five to eight times more likely to be affected than men. Yet many people live with undiagnosed thyroid problems for years, attributing their symptoms to stress, aging, or "just being tired."

This comprehensive guide explains thyroid function tests – TSH, Free T3, Free T4, and thyroid antibodies – in plain English. You'll learn what these tests measure, how to interpret your results, and what abnormal thyroid function means for your health.

What Does Your Thyroid Do?

Your thyroid gland produces hormones that regulate your metabolism – essentially the speed at which your body uses energy. These hormones affect nearly every organ and cell in your body:

• Energy production – How efficiently your cells convert food into energy
• Heart rate – How fast or slow your heart beats
• Body temperature – How warm you feel
• Weight management – How easily you gain or lose weight
• Digestive function – How quickly food moves through your system
• Muscle strength – How strong and coordinated your movements are
• Mood and mental clarity – Your cognitive function and emotional wellbeing
• Hair, skin, and nails – Their growth and quality
• Menstrual cycles – Regularity and heaviness of periods
• Fertility – Ability to conceive and maintain pregnancy

When your thyroid produces too much hormone (hyperthyroidism), everything speeds up. Too little hormone (hypothyroidism), and everything slows down.

Understanding the Thyroid Feedback Loop

Your thyroid doesn't work alone – it's part of a sophisticated feedback system:

1. Your hypothalamus (in your brain) releases TRH (thyrotropin-releasing hormone)
2. Your pituitary gland (also in your brain) responds by releasing TSH (thyroid-stimulating hormone)
3. Your thyroid gland responds to TSH by producing thyroid hormones:
   • T4 (thyroxine) – about 80% of production
   • T3 (triiodothyronine) – about 20% of production, but the more active hormone
4. Your body's cells convert T4 into T3 as needed
5. Feedback occurs: When thyroid hormone levels rise, TSH production decreases. When they fall, TSH production increases

This feedback loop is why TSH is usually the first test ordered – it's exquisitely sensitive to thyroid hormone levels and often the earliest indicator of thyroid dysfunction.

TSH: Thyroid Stimulating Hormone

TSH is produced by your pituitary gland and tells your thyroid to produce hormones. Think of TSH as your pituitary "shouting" at your thyroid to work harder or telling it to slow down.

Australian Reference Range: 0.5-4.0 mIU/L (some labs use 0.4-4.5 mIU/L)

However, there's ongoing debate about optimal TSH levels. Many endocrinologists believe the ideal range is narrower, around 0.5-2.5 mIU/L, especially for people with symptoms.

High TSH (Above 4.0 mIU/L)

What it means: Your pituitary is "shouting louder" because your thyroid isn't producing enough hormone. This indicates hypothyroidism (underactive thyroid).

Symptoms of hypothyroidism:

• Persistent fatigue and sluggishness
• Weight gain despite no change in diet
• Feeling cold when others are comfortable
• Dry skin and brittle hair or hair loss
• Constipation
• Muscle weakness and aches
• Depression or mood changes
• Brain fog and poor concentration
• Heavy or irregular menstrual periods
• Slowed heart rate
• Puffy face

Common causes:

• Hashimoto's thyroiditis – Autoimmune destruction of the thyroid (most common cause in Australia)
• Iodine deficiency – Rare in Australia since iodized salt was introduced, but can occur with very restricted diets
• Thyroid surgery or radioactive iodine treatment
• Certain medications – Lithium, amiodarone, some cancer drugs
• Pituitary disorders – Rarely
• Pregnancy and postpartum thyroiditis

Low TSH (Below 0.5 mIU/L)

What it means: Your pituitary has stopped "shouting" because your thyroid is producing too much hormone, or you're taking too much thyroid medication. This indicates hyperthyroidism (overactive thyroid) or over-treatment.

Symptoms of hyperthyroidism:

• Unintentional weight loss despite increased appetite
• Rapid or irregular heartbeat (palpitations)
• Feeling hot or sweating excessively
• Nervousness, anxiety, or irritability
• Trembling hands
• Difficulty sleeping
• Frequent bowel movements or diarrhea
• Muscle weakness, especially in upper arms and thighs
• Light or missed periods
• Bulging eyes (in Graves' disease)
• Increased heart rate (even at rest)

Common causes:

• Graves' disease – Autoimmune condition causing thyroid over-activity (most common cause)
• Toxic nodular goiter – Lumps in thyroid producing excess hormone
• Thyroiditis – Temporary inflammation releasing stored hormone
• Excessive thyroid medication
• Too much iodine – From supplements or medications like amiodarone

Subclinical Thyroid Dysfunction

Subclinical hypothyroidism: TSH is elevated (4.0-10 mIU/L) but T4 is still normal. You may or may not have symptoms.

Treatment depends on:

• How high your TSH is
• Whether you have symptoms
• Whether you have positive thyroid antibodies
• Your age and other health conditions
• Whether you're pregnant or trying to conceive

Many doctors will treat if TSH is above 10 mIU/L, or above 4-5 mIU/L if you have symptoms or positive antibodies.

Subclinical hyperthyroidism: TSH is low but thyroid hormones are still normal. This may require treatment if prolonged, as it increases risks of atrial fibrillation and osteoporosis.

Free T4 (Free Thyroxine)

T4 is the main hormone produced by your thyroid. "Free" T4 is the portion that's not bound to proteins and is available for your body to use. Most T4 is actually bound to proteins and inactive.

Australian Reference Range: 10-20 pmol/L (ranges vary between laboratories)

Low Free T4

Combined with high TSH: Confirms primary hypothyroidism – your thyroid itself is failing.

Combined with low or normal TSH: Suggests secondary hypothyroidism – the problem is with your pituitary gland, not your thyroid. This is rare and requires specialist investigation.

High Free T4

Combined with low TSH: Confirms hyperthyroidism – your thyroid is overproducing hormone.

Combined with high or normal TSH: Rare, but may indicate pituitary adenoma (TSH-secreting tumor), thyroid hormone resistance, or lab error.

Normal Free T4 with Abnormal TSH

This is subclinical thyroid dysfunction (described above). Your thyroid is still compensating, but the feedback loop is already disrupted.

Free T3 (Free Triiodothyronine)

T3 is the active thyroid hormone – it's 3-4 times more potent than T4. Most T3 is actually made by converting T4 to T3 in your liver, kidneys, and other tissues. Only about 20% comes directly from your thyroid.

Australian Reference Range: 3.5-6.5 pmol/L (ranges vary between laboratories)

T3 is not always tested initially because TSH and free T4 usually provide sufficient information. However, T3 is important in certain situations:

• T3 thyrotoxicosis – Rare form of hyperthyroidism where only T3 is elevated
• Monitoring hyperthyroidism treatment
• People on thyroid medication who still have symptoms – Some people don't convert T4 to T3 efficiently
• Non-thyroidal illness – Severe illness can suppress T3 without true thyroid disease

Low T3

With hypothyroidism: Confirms significant thyroid hormone deficiency.

Low T3 syndrome (euthyroid sick syndrome): T3 drops during severe illness, starvation, or extreme stress as a protective mechanism. This usually doesn't require thyroid medication and resolves when the underlying condition improves.

Conversion problems: Some people have genetic variations or nutrient deficiencies (selenium, zinc, iron) that impair T4 to T3 conversion.

High T3

Combined with low TSH and high T4, confirms hyperthyroidism. In rare cases, T3 may be elevated while T4 is normal (T3 toxicosis).

Thyroid Antibodies

Thyroid antibody tests determine if autoimmune disease is causing your thyroid dysfunction. In Australia, autoimmune thyroid disease is the most common cause of both hypothyroidism and hyperthyroidism.

Thyroid Peroxidase Antibodies (TPO Antibodies)

TPO antibodies attack an enzyme in your thyroid essential for hormone production. High levels indicate Hashimoto's thyroiditis, the most common cause of hypothyroidism in Australia.

Normal: Less than 35 IU/mL (varies by lab)

Positive TPO antibodies mean:

• You have or will likely develop hypothyroidism
• Subclinical hypothyroidism is more likely to progress
• You're at higher risk of postpartum thyroiditis
• You may benefit from thyroid medication even with mild TSH elevation

About 10-15% of people without thyroid disease also have positive TPO antibodies, particularly women. This increases their risk of future thyroid problems.

Thyroglobulin Antibodies (TgAb)

These antibodies attack thyroglobulin, a protein your thyroid uses to make hormones. Often elevated with TPO antibodies in Hashimoto's thyroiditis.

Normal: Less than 40 IU/mL (varies by lab)

TSH Receptor Antibodies (TRAb or TSI)

These antibodies mimic TSH and stimulate your thyroid to overproduce hormone. Positive TRAb indicates Graves' disease, the most common cause of hyperthyroidism.

Normal: Less than 1.8 IU/L (varies by lab)

TRAb testing is particularly important for:

• Diagnosing Graves' disease vs. other causes of hyperthyroidism
• Pregnant women with current or past Graves' disease (antibodies can cross placenta and affect baby)
• Predicting relapse after stopping anti-thyroid medication

Thyroid Disorders in Detail

Hashimoto's Thyroiditis

Hashimoto's is an autoimmune disease where your immune system attacks your thyroid, gradually destroying it. It's the most common thyroid disorder in Australia, affecting about 1-2% of the population, with women 10 times more likely to develop it than men.

Progression:

1. Initially, thyroid function may be normal (euthyroid) with positive antibodies
2. TSH rises (subclinical hypothyroidism) but T4 remains normal
3. Eventually, T4 drops (overt hypothyroidism)
4. Some people experience "Hashitoxicosis" – temporary hyperthyroidism as damaged cells release stored hormone

Risk factors:

• Female sex
• Family history of thyroid or autoimmune disease
• Other autoimmune conditions (type 1 diabetes, celiac disease, vitiligo, etc.)
• Pregnancy (postpartum thyroiditis)
• Viral infections
• High iodine intake
• Radiation exposure

Treatment: Levothyroxine (synthetic T4) is the standard treatment. Most people need medication for life. Dose requirements may change over time as thyroid function continues to decline.

Graves' Disease

Graves' disease is an autoimmune condition where antibodies stimulate your thyroid to produce excessive hormone. It affects about 0.5% of Australians, with women 5-10 times more likely to develop it.

Unique features of Graves' disease:

• Graves' ophthalmopathy (eye disease): Bulging eyes, eye pain, double vision (30-50% of patients)
• Pretibial myxedema: Skin thickening on shins (rare)
• Diffuse goiter: Entire thyroid gland enlarges

Treatment options:

1. Anti-thyroid medications: Carbimazole or propylthiouracil (PTU) – usually given for 12-18 months with 30-50% remission rate after stopping
2. Radioactive iodine ablation: Destroys overactive thyroid tissue – usually results in hypothyroidism requiring lifelong levothyroxine
3. Surgery (thyroidectomy): Removal of thyroid gland – reserved for large goiters, pregnancy planning, or failure of other treatments

Thyroid Nodules and Goiter

Thyroid nodules are very common, found in up to 50% of Australians by age 60. Most are benign, but about 5-15% are cancerous.

When nodules require investigation:

• Nodule larger than 1 cm
• Rapid growth
• Difficulty swallowing or breathing
• Hoarse voice
• Family history of thyroid cancer
• Previous radiation exposure to head/neck
• Abnormal thyroid function tests

Investigation typically involves thyroid ultrasound and possibly fine needle aspiration (FNA) biopsy.

Thyroid Health and Special Populations

Pregnancy and Thyroid Function

Thyroid hormones are crucial for fetal brain development, especially in the first trimester before the baby's thyroid develops. Both hypothyroidism and hyperthyroidism can cause pregnancy complications:

Untreated hypothyroidism risks:

• Miscarriage
• Preterm birth
• Low birth weight
• Preeclampsia
• Impaired fetal brain development

Untreated hyperthyroidism risks:

• Miscarriage
• Preterm birth
• Low birth weight
• Preeclampsia
• Fetal or neonatal hyperthyroidism (if caused by Graves' disease)

Thyroid screening in pregnancy:

• Women with known thyroid disease should have TSH checked before conception and every 4-6 weeks during pregnancy
• Universal screening isn't routinely recommended, but testing is appropriate if you have symptoms or risk factors
• TSH targets are lower in pregnancy: <2.5 mIU/L in first trimester, <3.0 mIU/L in second and third trimesters
• Levothyroxine requirements typically increase by 25-50% during pregnancy

Postpartum Thyroiditis

About 5-10% of women develop postpartum thyroiditis in the year after delivery. It typically follows a pattern:

1. Hyperthyroid phase (1-4 months postpartum): Anxiety, palpitations, weight loss, insomnia
2. Hypothyroid phase (4-8 months postpartum): Fatigue, depression, weight gain, hair loss
3. Resolution: 80% recover normal thyroid function, but 20% develop permanent hypothyroidism

Postpartum thyroiditis is often misdiagnosed as postpartum depression. Women with positive thyroid antibodies before or during pregnancy are at highest risk.

Older Australians

Thyroid disorders become more common with age:

• Subclinical hypothyroidism affects 10-15% of people over 65
• Symptoms may be subtle and attributed to "normal aging"
• Treatment thresholds may be higher in elderly (some studies suggest TSH up to 6-7 may be normal after age 70)
• Start levothyroxine at lower doses and increase slowly to avoid cardiac stress

Lifestyle and Thyroid Health

Iodine: Too Little or Too Much

Iodine is essential for thyroid hormone production, but Australia has a complex history with iodine:

• Historically iodine deficient: Tasmania had the highest goiter rates in the world in the 1940s-50s
• Iodization improved status: Mandatory iodization of bread (2009) reduced but didn't eliminate deficiency
• Pregnant women at risk: Requirements increase 50% during pregnancy; prenatal supplements recommended
• Vegan/vegetarian diets: May be lower in iodine (sea vegetables vary widely and can provide too much)

Iodine-rich foods:

• Iodized salt (use moderately)
• Seafood, especially fish and shellfish
• Dairy products
• Eggs
• Bread (in Australia, fortified with iodized salt)

Caution: Excessive iodine from supplements or kelp can trigger thyroid dysfunction, especially in people with underlying thyroid disease.

Selenium

Selenium is crucial for converting T4 to active T3 and protecting the thyroid from oxidative damage. Australia has selenium-poor soils (except in parts of Queensland and South Australia), so deficiency is possible.

Selenium sources:

• Brazil nuts (2-3 daily provide adequate selenium – don't overdo it)
• Seafood
• Meat and poultry
• Eggs

Some studies suggest selenium supplementation may reduce thyroid antibodies in Hashimoto's thyroiditis, though more research is needed.

Stress and Thyroid Function

Chronic stress can affect thyroid function through:

• Disrupting the hypothalamic-pituitary-thyroid axis
• Impairing T4 to T3 conversion
• Increasing reverse T3 (inactive form)
• Exacerbating autoimmune thyroid disease

Stress management – adequate sleep, regular exercise, mindfulness – supports thyroid health.

Goitrogens: Foods That Affect Thyroid Function

Goitrogens are substances that can interfere with thyroid function by blocking iodine uptake. However, they only cause problems if you're iodine deficient or eat them in excessive amounts.

Goitrogenic foods:

• Cruciferous vegetables: broccoli, cauliflower, Brussels sprouts, cabbage, kale
• Soy products
• Cassava

Important note: If you have adequate iodine intake and eat these foods cooked (cooking deactivates most goitrogens), they're not a problem. Don't avoid these nutritious foods unless your doctor specifically advises it.

When to Test Your Thyroid

You should request thyroid testing if you experience:

• Unexplained fatigue or weight changes
• Feeling excessively cold or hot
• Hair loss or skin changes
• Menstrual irregularities or infertility
• Mood changes, depression, or anxiety
• Heart palpitations or changes in heart rate
• Muscle weakness or tremors
• Changes in bowel habits
• Difficulty concentrating
• Visible neck swelling

Regular screening recommended for:

• Women over 50
• Family history of thyroid disease
• Personal history of autoimmune disease
• Type 1 diabetes
• Previous thyroid problems or treatment
• Previous radiation to head/neck
• Taking medications that affect thyroid (lithium, amiodarone)
• Pregnant or planning pregnancy
• Postpartum period

Resources for Further Reading

Conclusion

Your thyroid may be small, but it's mighty. Understanding your thyroid function tests empowers you to recognize problems early and work effectively with your doctor to optimize your treatment.

Thyroid disorders are highly treatable. With proper diagnosis and management, most people with thyroid problems can feel completely normal and enjoy excellent quality of life. The key is recognizing symptoms, getting tested, and finding the right treatment approach for you.

Remember that thyroid function can change over time. If you're on thyroid medication, regular monitoring ensures your dose remains optimal. If you have positive antibodies but normal function now, monitoring can catch progression early.

By tracking your thyroid test results over time with BloodResults, you can see trends that might not be apparent from isolated tests. This longitudinal view helps you and your doctor make the best decisions for your thyroid health.