Hypothyroidism means the thyroid — the small gland at the base of the neck — makes too little thyroid hormone, and because that hormone sets the pace of nearly every organ, the whole body slows down. Heart rate, metabolism, digestion, mood and temperature all ease off together, which is why the effects are so wide-ranging and so easy to mistake for stress or simple ageing. It is common, affecting a few percent of adults and far more women than men, and the risk climbs with age. In countries with enough dietary iodine, the usual cause is Hashimoto’s, an autoimmune attack on the gland. It is confirmed with a blood test: a raised TSH, usually alongside a low free T4, is enough to make the diagnosis.
Who is at risk
The thyroid falters more often in some people than others. The main risk factors are:
- Being female and older — women are several times more likely to be affected, and prevalence rises after 60.
- Autoimmune disease, personal or in the family: type 1 diabetes, celiac disease, vitiligo, rheumatoid arthritis, or a relative with thyroid disease.
- Pregnancy and the year after it, when postpartum thyroiditis can appear.
- Previous thyroid treatment — surgery, radioactive iodine, or radiation to the neck.
- Certain medicines, including lithium, amiodarone and some cancer immunotherapies.
- Iodine imbalance — too little or, sometimes, too much.
Symptoms
Thyroid symptoms are famously vague and easy to blame on a busy life. The most common are persistent fatigue, feeling cold when others are comfortable, unexplained weight gain, constipation, dry skin, and diffuse hair thinning or brittle nails. Many people notice low mood, slowed thinking or “brain fog,” heavier or irregular periods, muscle aches, and a general puffiness — sometimes visible as swelling around the eyes, face or legs. In more advanced cases the voice can turn hoarse and the heart rate slows. Slowed nerves can cause tingling or numbness in the hands and feet. Because this picture overlaps so closely with iron deficiency, the two are easily mixed up in women; our guide to iron deficiency versus hypothyroidism shows how the labs separate them. If you are starting from a symptom rather than a diagnosis, the symptoms hub links each complaint to its test checklist.
Which lab tests confirm it
Thyroid testing is a short, logical sequence, and one well-timed blood draw usually settles it — unlike many conditions, the numbers map cleanly onto the diagnosis.
TSH comes first and does most of the work. TSH is the pituitary’s instruction to the thyroid; when the gland underperforms, the pituitary shouts louder, so TSH rises early — often before any other test turns abnormal. That sensitivity makes it the single best screening and monitoring test. Most labs place the reference range near 0.4–4.0 mIU/L, though the upper limit drifts up with age.
Free T4 measures the main hormone the thyroid releases and confirms how far things have gone. A high TSH with a low free T4 is overt hypothyroidism; a high TSH with a still-normal free T4 is the milder, subclinical form.
Free T3 is the active hormone tissues actually use, but the body defends its level, so it stays normal until late and adds little to the diagnosis of an underactive thyroid — it is ordered selectively rather than routinely, and a low free T3 far more often reflects another illness than a thyroid problem.
TPO antibodies reveal the cause. Raised anti-TPO points to Hashimoto’s autoimmune thyroiditis and, per the ATA, helps predict who with a borderline TSH will progress to overt disease.
Order of testing: TSH alone is the usual starting point. If it is abnormal, the lab reflexes to free T4, and TPO antibodies are added to establish an autoimmune cause. An abnormal TSH is repeated before treatment, since it can shift with illness. Two practical traps are worth knowing: a serious non-thyroidal illness can temporarily disturb thyroid numbers, so testing is best done when you are otherwise well, and high-dose biotin supplements can skew many thyroid immunoassays, so they are usually paused for a couple of days beforehand.
How to read the results together
- Overt hypothyroidism: high TSH + low free T4. The gland cannot keep up, and treatment is usually recommended.
- Subclinical hypothyroidism: high TSH (often 4.5–10 mIU/L) + normal free T4. Whether to treat or monitor depends on the exact TSH, symptoms, TPO status and pregnancy plans — the higher the TSH and the more symptoms or antibodies, the stronger the case to treat.
- Hashimoto’s: any of the above plus positive TPO antibodies — the commonest underlying cause and a signal of higher risk of progression.
- A low or normal TSH with a low free T4 suggests the problem lies in the pituitary rather than the thyroid itself — central hypothyroidism. It is uncommon but changes the work-up, so it is referred.
- A high TSH with a high free T4 does not fit primary hypothyroidism and points elsewhere, so it prompts specialist review rather than routine treatment.
What happens next
If a first abnormal TSH holds up on a repeat, treatment replaces the missing hormone with levothyroxine, a synthetic version of T4 taken once a day. It is best absorbed on an empty stomach, well apart from coffee and from iron or calcium supplements, which otherwise blunt its uptake. The dose is individual — set by your doctor and adjusted to how you feel and to your TSH, never guessed — so this page gives no doses. Because TSH takes time to respond, it is usually rechecked about 6–8 weeks after starting or changing the dose, then yearly once stable. Improvement is gradual — energy, mood and weight tend to shift over weeks to a few months as levels settle rather than overnight, so a little patience during the settling-in period matters. Subclinical hypothyroidism is sometimes watched rather than treated, with a repeat in a few months. Pregnancy and trying to conceive change the targets and need prompt specialist input. Lifestyle steps do not cure an underactive thyroid, but treating any co-existing iron or vitamin D shortfall can help the lingering fatigue. Most hypothyroidism from Hashimoto’s is permanent, so treatment is usually lifelong — but it is straightforward once the dose is right, and the great majority of people feel fully back to normal. If symptoms persist despite a normal TSH, it is worth looking for another explanation — iron deficiency, low vitamin D, poor sleep or low mood — rather than pushing the dose higher, which rarely helps and can cause harm.
When to see a doctor
Most hypothyroidism is managed calmly in primary care, but a few situations need urgency. Seek prompt help for a rapidly enlarging neck lump or trouble swallowing or breathing. Extreme drowsiness, confusion, very low body temperature or a slow heartbeat in someone with known thyroid disease is a medical emergency. Anyone pregnant or planning pregnancy with thyroid symptoms should be assessed early, as should new symptoms after starting a drug like lithium or amiodarone. Newborns are screened for thyroid problems at birth because early hormone is vital for brain development, and a child who is growing or developing slowly deserves a check too.
