Thyroid Function Tests Explained: TSH, T4, and T3

Thyroid test results confuse people in a specific way: the main marker, TSH, moves in the opposite direction to what you'd intuitively expect. If your thyroid is underactive, TSH goes up — not down. If your thyroid is overactive, TSH goes down — not up. Once you understand why, the results start to make complete sense.

What Does the Thyroid Gland Actually Do?

The thyroid is a butterfly-shaped gland in your neck that produces hormones controlling your body's metabolic rate — how fast or slow cells work. Thyroid hormone (predominantly thyroxine, or T4) influences virtually every organ system: heart rate, body temperature, energy levels, bowel function, mood, weight, and fertility.

The system is regulated by a feedback loop. Your pituitary gland (at the base of the brain) monitors thyroid hormone levels in the blood. When they drop, the pituitary secretes Thyroid Stimulating Hormone (TSH) to tell the thyroid to produce more. When thyroid hormone levels rise, TSH falls to slow production down. Think of TSH as the thermostat dial — it turns up when the room is cold (low thyroid hormone) and down when it's hot (high thyroid hormone).

Understanding TSH — The Most Important Number

TSH is almost always the first test ordered when thyroid disease is suspected, and for most people it's sufficient on its own for monitoring. The normal range in adults is approximately 0.4–4.0 mIU/L, though this varies slightly by laboratory.

T4 and T3 — The Actual Thyroid Hormones

The thyroid gland mainly produces thyroxine (T4), which is then converted in the body's tissues to the more active form, triiodothyronine (T3). Tests measure either "total" or "free" levels — free T4 (fT4) and free T3 (fT3) measure only the active, unbound hormone and are more clinically useful.

In NHS practice, most GPs start with TSH alone. If TSH is abnormal, free T4 is added. Free T3 is less commonly tested routinely but is useful when hyperthyroidism is suspected or when monitoring T3-based replacement therapy.

Thyroid Result Patterns Explained

TSH	Free T4	Meaning
Normal (0.4–4.0)	Normal	Thyroid function is healthy. No action needed.
High (>4.0)	Normal	Subclinical hypothyroidism. The thyroid is compensating but under strain. Often monitored; may need treatment.
High (>4.0)	Low	Overt hypothyroidism. Underactive thyroid — levothyroxine replacement is typically started.
Low (<0.4)	Normal	Subclinical hyperthyroidism. May need monitoring or treatment depending on symptoms and cause.
Low (<0.4)	High	Overt hyperthyroidism. Overactive thyroid — requires further investigation and treatment.

Hypothyroidism — The Underactive Thyroid

Hypothyroidism is one of the most common endocrine conditions in the UK, affecting around 2% of the population — the vast majority women. The most common cause is Hashimoto's thyroiditis, an autoimmune condition in which the immune system gradually destroys thyroid tissue.

Symptoms include fatigue, weight gain, feeling cold, constipation, dry skin, hair loss, low mood, slow heart rate, and heavy or irregular periods. However, many people have few or no symptoms even with significantly abnormal TSH — which is why blood tests are so important.

Treatment is with levothyroxine — a synthetic form of T4 taken daily. The dose is adjusted over time based on repeat TSH levels. Most people feel significantly better within 6–8 weeks of starting treatment, though some symptoms (particularly fatigue) may take longer to resolve. Once stable, monitoring is typically annual.

Hyperthyroidism — The Overactive Thyroid

The most common cause of an overactive thyroid in the UK is Graves' disease — another autoimmune condition, in which antibodies stimulate the thyroid to produce excess hormone. Other causes include toxic multinodular goitre and thyroiditis (inflammation of the thyroid).

Symptoms include weight loss despite good appetite, palpitations, heat intolerance, sweating, anxiety, tremor, frequent bowel movements, and in Graves' disease specifically, eye changes (proptosis). Treatment options include antithyroid medications (carbimazole or propylthiouracil), radioiodine ablation, or thyroidectomy (surgery to remove the gland).

Thyroid Antibodies — What Are They?

If your GP suspects an autoimmune cause, they may order thyroid peroxidase antibodies (TPO-Ab) or thyroid stimulating immunoglobulins (TSI). Elevated TPO antibodies strongly suggest Hashimoto's thyroiditis. TSI (or TRAb) antibodies are specific to Graves' disease. These tests help identify the underlying cause and guide management.

What About "Borderline" or "Subclinical" Results?

A mildly elevated TSH with a normal fT4 — often called "subclinical hypothyroidism" — is a common finding and a common source of anxiety. Whether to treat depends on the TSH level, symptoms, age, whether the person is planning pregnancy, and whether antibodies are positive. In younger people with symptoms and positive antibodies, treatment is often started. In older adults with a mildly raised TSH and no symptoms, watchful waiting with repeat testing is usually preferred.

If you've received a borderline result, the most important next step is a conversation with your GP about the clinical context — not just the number.

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