Interpreting Thyroid Function Tests
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Correct interpretation of Thyroid function tests (TFTs) is essential to avoid misdiagnosis and inappropriate treatment.
Key Tests
- TSH (thyroid-stimulating hormone): Most sensitive marker of thyroid dysfunction.
- Free T4 (fT4): Reflects circulating thyroxine not bound to proteins.
- Free T3 (fT3): Typically measured when hyperthyroidism is suspected.
| TSH | Free T4 | Free T3 | Likely Diagnosis | Notes |
|---|---|---|---|---|
| ↑ | ↓ | – | Primary hypothyroidism | Most common; treat with levothyroxine |
| ↑ | Normal | – | Subclinical hypothyroidism | Monitor or treat depending on age, symptoms, TPO Abs, pregnancy |
| ↓ | ↑ | ↑ or Normal | Primary hyperthyroidism | Graves’, toxic nodular goitre; consider endocrinology referral |
| ↓ | Normal | Normal | Subclinical hyperthyroidism | Monitor; treat if TSH <0.1 or risk factors (AF, osteoporosis, age >65) |
| ↓ or Normal | ↓ | – | Central (secondary) hypothyroidism | Pituitary cause; urgent referral |
| Normal | Normal | Normal | Euthyroid | No thyroid dysfunction |
| ↓ | ↓ or Normal | ↓ | Non-thyroidal illness (sick euthyroid) | TFTs altered by acute illness; repeat when well |
Primary Hypothyroidism
- TSH raised, fT4 low: Classic picture of primary hypothyroidism.
- Common causes: Hashimoto’s thyroiditis, post-radioiodine or thyroidectomy.
- Treat with levothyroxine, titrating to normal TSH.
Subclinical Hypothyroidism
- TSH raised, fT4 normal.
- Consider treatment if:
- TSH >10 mU/L
- Symptomatic
- Positive thyroid peroxidase (TPO) antibodies
- Planning pregnancy or already pregnant
- Monitor 6–12 monthly if not treated.
Primary Hyperthyroidism
- TSH suppressed, fT4 and/or fT3 raised.
- Common causes: Graves’ disease, toxic multinodular goitre.
- Consider referral for radioiodine, antithyroid drugs or surgery.
Subclinical Hyperthyroidism
- TSH suppressed, fT4 and fT3 normal.
- Often asymptomatic.
- Risks: atrial fibrillation, osteoporosis.
- Consider treatment or endocrinology referral if:
- Age >65
- TSH persistently <0.1 mU/L
- Underlying heart disease or osteoporosis
Sick Euthyroid Syndrome
- Abnormal TFTs in acute illness (often low T3).
- Do not treat; repeat when patient is well.
Central Hypothyroidism
- TSH low or normal, fT4 low.
- Suggests pituitary or hypothalamic dysfunction.
- Urgent endocrinology referral required.
Monitoring Treatment
- Levothyroxine: Monitor TSH every 3 months initially, then 12 monthly once stable.
- Antithyroid drugs: Monitor fT4 and TSH; beware TSH may remain suppressed for months.