Subclinical hyperthyroidism: low TSH with normal T4, risks and when to treat
Subclinical hyperthyroidism means a low TSH with normal free T4 and T3. What causes it, the heart and bone risks, when guidelines suggest treatment with carbimazole or radioiodine, and how often to recheck.
Subclinical hyperthyroidism is a blood test finding: TSH is below the reference range, but free T4 and free T3 are normal. The thyroid is running slightly fast, enough for the pituitary to turn down its signal but not enough to push hormone levels out of range. Many people feel fine. The question is whether, and when, it needs treatment such as carbimazole.
Key points
- Definition (NICE): TSH below the reference range, with free T4 and T3 within it.
- Two grades: TSH 0.1 to 0.39 (grade 1) and below 0.1 (grade 2), as used by the European Thyroid Association.
- Risks: pooled studies link it with atrial fibrillation, heart disease deaths and fractures, mainly at TSH below 0.1.
- Treatment: usually advised for people over 65 with TSH below 0.1; often just monitoring for younger people without symptoms.
Causes
The European Thyroid Association lists Graves’ disease, single overactive nodules and multinodular goitre as the causes of the “endogenous” form. A low TSH can also come from taking too much levothyroxine, which is managed by adjusting that dose, and a temporary low TSH can follow thyroiditis. Because results can bounce back, a single low TSH should be repeated before anything is decided.
Why it matters
Two large pooled analyses put numbers on the risk. Across 10 cohorts and 52,674 people, subclinical hyperthyroidism was linked with higher total mortality (hazard ratio 1.24), more deaths from coronary heart disease (1.29) and more atrial fibrillation (1.68), with the heart risks highest when TSH was below 0.1. Across 13 cohorts and 70,298 people, it was linked with more hip fractures (hazard ratio 1.36) and more fractures overall (1.28). These are associations, not proof that treatment prevents them; there are no randomised trials.
When to treat
The 2015 European Thyroid Association guideline, in summary:
| TSH | Age 65 or over | Under 65 |
|---|---|---|
| Below 0.1 (grade 2) | Treatment indicated | Treatment reasonable, especially with symptoms or heart or bone risk |
| 0.1 to 0.39 (grade 1) | Treatment could be considered (atrial fibrillation risk) | No treatment; monitor |
NICE in the UK suggests seeking specialist advice for adults with two TSH readings below 0.1 at least three months apart, together with evidence of thyroid disease (a goitre or positive antibodies) or symptoms; for children and young people, specialist advice in all cases.
When treatment is chosen, the options are those for overt hyperthyroidism: an antithyroid drug such as carbimazole or methimazole, radioactive iodine, or surgery, depending on the cause. See hyperthyroidism and radioactive iodine.
Monitoring without treatment
NICE suggests checking TSH every six months in adults with untreated subclinical hyperthyroidism, adding free T4 and T3 if TSH is out of range, and every three months in children. Testing can stop once TSH is stable within the range on two checks three to six months apart.
If you are prescribed carbimazole
Your doctor sets the dose from your blood tests. Carbimazole is listed by PockDose, Yasabai and VGR Australia. See dosage and blood tests.
Sources
Checked 15 October 2026.
- Thyroid disease: assessment and management (NG145), recommendations 1.8.1 to 1.8.5 and terms — National Institute for Health and Care Excellence
- The 2015 European Thyroid Association guidelines on endogenous subclinical hyperthyroidism (Biondi et al., 2015) — European Thyroid Journal
- Subclinical hyperthyroidism and the risk of coronary heart disease and mortality (Collet et al., 2012) — Archives of Internal Medicine
- Subclinical thyroid dysfunction and fracture risk, a meta-analysis (Blum et al., 2015) — JAMA