Thyroid function tests look simple: one TSH and one free T4, occasionally a free T3. Yet they generate some of the most discriminating questions in the Endocrinology and Diabetes SCE and in MRCP, because the answer usually depends on reading the results together and placing them in the clinical context. This overview is written for physicians preparing for exams and for those refreshing their knowledge.

Read the pair, not the single value

TSH is the most sensitive marker of primary thyroid disease, because a small change in free T4 produces a large change in TSH. That is why TSH alone is often used for screening, and why a normal TSH with an abnormal free T4 should always make you pause.

The common patterns

  • High TSH with low free T4 suggests primary hypothyroidism, most often autoimmune in the UK.
  • High TSH with normal free T4 is described as subclinical hypothyroidism.
  • Low TSH with high free T4 or free T3 suggests primary hyperthyroidism, such as Graves' disease or toxic nodular goitre.
  • Low TSH with normal free hormones is described as subclinical hyperthyroidism.
  • A low or inappropriately normal TSH with low free T4 points to central disease of the pituitary or hypothalamus.
  • A raised free T4 with a normal or raised TSH raises the possibility of assay interference, a TSH-secreting pituitary adenoma or resistance to thyroid hormone.

Non-thyroidal illness

In acutely unwell patients, TSH and free T4 can both fall and free T3 is often low. This pattern, sometimes called sick euthyroid syndrome, reflects the illness rather than thyroid disease, and exams test whether you recognise it.

Medicines and interference

Several medicines affect thyroid function, including amiodarone and lithium, and some supplements, notably biotin, interfere with laboratory assays. When the numbers do not fit the patient, the drug and supplement history is often the key.

Pregnancy

Normal pregnancy changes thyroid physiology. hCG stimulates the TSH receptor in the first trimester and binding proteins rise, so trimester-specific reference ranges are used.

How to revise this topic

Write each pattern on a card and practise naming the likely cause before checking the answer. In the exam, read the stem for medicines, pregnancy and acute illness before you choose.