The thyroid is a small butterfly-shaped gland at the base of your neck that produces hormones controlling metabolism, heart rate, body temperature, mood, and energy levels. Thyroid disorders — both overactive (hyperthyroidism) and underactive (hypothyroidism) — are extremely common, affecting roughly 5–10% of the population, with higher rates in women and older adults.
Thyroid function is assessed with blood tests that measure hormone levels in a cascade: the brain signals the thyroid via TSH, and the thyroid responds with T4 and T3. Understanding this chain makes the results far easier to interpret.
Thyroid blood tests measure the hormones that regulate your metabolism, energy and many other body systems. The core panel — TSH, free T4 and sometimes free T3 — tells clinicians whether the thyroid is overactive (hyperthyroidism), underactive (hypothyroidism) or functioning normally, and helps identify the underlying cause when results are abnormal.
TSH: the most important single measurement
TSH (thyroid-stimulating hormone) is produced by the pituitary gland in the brain. It tells the thyroid how much hormone to make. When thyroid hormone levels are low, the pituitary produces more TSH to stimulate the thyroid. When levels are high, it produces less.
This inverse relationship is why TSH is the first-line screening test for thyroid disorders: a high TSH usually means an underactive thyroid; a low TSH usually means an overactive thyroid.
- **Normal range:** approximately 0.4–4.0 mU/L (this varies slightly between laboratories)
- **High TSH (above 4.0 mU/L):** suggests hypothyroidism — the thyroid is underperforming and the pituitary is working harder to compensate
- **Low TSH (below 0.4 mU/L):** suggests hyperthyroidism — the thyroid is overproducing, suppressing the pituitary's output
- **Very low or undetectable TSH:** associated with significant hyperthyroidism or over-treatment with thyroid medication
"Thyroid results are almost always a story that unfolds over several tests. A single reading rarely tells you enough. Track the trend, and context becomes everything."
Dr. Lina Haddad, CMO, Looms
Free T4 (fT4): the main thyroid hormone
T4 (thyroxine) is the primary hormone produced by the thyroid. 'Free T4' refers to the fraction not bound to proteins in the blood — the portion available to act on cells. T4 is relatively inactive itself and gets converted to the more potent T3 in tissues.
- **Normal range:** approximately 9–23 pmol/L (varies by laboratory and assay)
- **Low fT4 with high TSH:** confirms primary hypothyroidism — the thyroid is not responding sufficiently to the pituitary's signal
- **High fT4 with low TSH:** confirms hyperthyroidism — the thyroid is overproducing, suppressing TSH
- **Low fT4 with low TSH:** suggests secondary hypothyroidism — a pituitary or hypothalamic problem rather than a thyroid problem, a less common pattern requiring specialist evaluation
Free T3 (fT3): the active hormone
T3 (triiodothyronine) is the biologically active form of thyroid hormone. Most T3 is produced in the body's tissues by converting T4, rather than directly by the thyroid. Free T3 is not routinely ordered in the initial investigation of thyroid problems — it's most useful when hyperthyroidism is suspected but T4 is normal ('T3 toxicosis'), or to monitor patients taking T3-containing replacement therapy.
Thyroid antibodies
If thyroid function is abnormal, or in some cases even when it's normal but symptoms persist, doctors test for thyroid antibodies to identify the underlying cause:
- **TPO antibodies (anti-thyroid peroxidase):** elevated in Hashimoto's thyroiditis, the most common cause of hypothyroidism in countries with adequate iodine intake. Also elevated in Graves' disease.
- **TRAb (TSH receptor antibodies):** elevated in Graves' disease, the most common cause of hyperthyroidism. These antibodies mimic TSH and stimulate the thyroid to produce excess hormone.
- **Anti-thyroglobulin antibodies:** less specific; elevated in both Hashimoto's and Graves'.
Subclinical thyroid disease: what 'borderline' means
Subclinical hypothyroidism is defined as a raised TSH with a normal T4. Many people with subclinical hypothyroidism have no symptoms; others have mild fatigue, weight gain, or mood changes. Whether to treat it is a clinical judgement that depends on the TSH level, the presence of antibodies, symptoms, age and other factors.
Subclinical hyperthyroidism — low TSH with normal T4 and T3 — similarly requires clinical context to interpret. In older adults, even mild TSH suppression carries an increased risk of atrial fibrillation and bone loss.
If you're on levothyroxine, these targets matter
The most commonly prescribed medication in the UK and one of the most prescribed globally, levothyroxine replaces T4 in people with hypothyroidism. Target TSH for most people on treatment is 0.5–2.5 mU/L. The dose is adjusted based on serial TSH measurements, typically every 6–12 weeks after any change until levels stabilise, then annually.
It's important to take levothyroxine consistently — the same time each day, ideally on an empty stomach — because absorption is significantly affected by food, coffee, calcium and iron supplements. If your TSH is unexpectedly variable, inconsistent dosing timing is a common reason.
Tracking thyroid function over time
Thyroid function tests are almost always interpreted as a trend rather than a single reading. A TSH of 5.2 that has been stable for three years means something very different from a TSH of 5.2 that has risen from 1.8 over eighteen months. Looms' lab trends feature charts your thyroid values automatically across all tests, so the direction of change is immediately visible.