The four patterns
| TSH | Free T4 | Interpretation |
|---|---|---|
| High | Low | Primary hypothyroidism |
| High | Normal | Subclinical hypothyroidism |
| Low | High | Hyperthyroidism |
| Low | Normal | Subclinical hyperthyroidism |
Two patterns that do not fit and matter:
- Low or normal TSH with low free T4 → central (secondary) hypothyroidism. Pituitary or hypothalamic disease. Check the rest of the pituitary axis, and replace cortisol before thyroxine — replacing thyroxine first can precipitate an adrenal crisis
- High TSH with high free T4 → TSH-secreting adenoma or thyroid hormone resistance, both rare, or assay interference
Hypothyroidism
Commonest cause is Hashimoto's thyroiditis — anti-TPO antibodies positive. Also: post-ablative, drugs (amiodarone, lithium), iodine deficiency worldwide.
- Levothyroxine, taken on an empty stomach, separated from calcium, iron, and proton pump inhibitors
- Recheck TSH 6–8 weeks after any dose change — sooner is uninterpretable
- Start low (25–50 µg) in the elderly and in ischaemic heart disease
- Pregnancy increases requirements by about 30% from early in the first trimester, and the TSH target is trimester-specific and lower
Subclinical hypothyroidism: treat if TSH > 10, if symptomatic with positive anti-TPO, or in pregnancy or those trying to conceive. Otherwise recheck.
Hyperthyroidism
Distinguish the causes by uptake, because it changes treatment entirely:
- High uptake, diffuse → Graves' disease (TSH receptor antibodies, ophthalmopathy, pretibial myxoedema)
- High uptake, focal → toxic adenoma or multinodular goitre
- Low uptake → thyroiditis (subacute/de Quervain's — painful, post-viral; silent; postpartum) or exogenous thyroid hormone. These are self-limiting and antithyroid drugs do not work
Treatment of Graves':
- Beta-blocker for symptoms immediately
- Methimazole first-line, except in the first trimester, where propylthiouracil is preferred because methimazole is teratogenic
- Monitor for agranulocytosis — any sore throat or fever means stop the drug and check the neutrophil count urgently — and hepatotoxicity, particularly with propylthiouracil
- Radioactive iodine or surgery as definitive options. Radioiodine may worsen ophthalmopathy and is contraindicated in pregnancy and breastfeeding
Sick euthyroid syndrome
In any seriously unwell inpatient, T3 falls first, then T4, with a variable TSH that rises during recovery. It is not thyroid disease. The lesson is not to test thyroid function in acutely unwell inpatients without a clear indication — a very examinable point.
Thyroid nodules
Ultrasound and TSH. If TSH is suppressed, scintigraphy first — a hot nodule is essentially never malignant. Otherwise fine-needle aspiration based on size and sonographic features.