27 Mar 2025•Dr Zaid Ismail
Interpreting Thyroid Function Tests
Correct interpretation of Thyroid function tests (TFTs) is essential to avoid misdiagnosis and inappropriate treatment. Key Tests TSH Free T4 Free T3 Likely Diagnosis Notes ↑ ↓ – Primary hypothyroidism Most common; treat with...
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 |
Summary table comparing the common thyroid function test (TFT) patterns and their clinical interpretation
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.