Thyroid function — Frequently asked questions (FAQ)
1) When should thyroid function be assessed?
Thyroid function assessment is indicated in the presence of symptoms suggestive of hypothyroidism or hyperthyroidism, goitre or thyroid nodules, infertility, menstrual irregularities, autoimmune diseases, dyslipidaemia, arrhythmias, osteoporosis, unexplained anaemia, a family history of thyroid disease or a history of neck irradiation. It should also be considered during pregnancy or in women planning to conceive.
2) What are the most important initial laboratory tests?
Initial assessment usually includes measurement of TSH and free T4 (FT4). When hyperthyroidism is suspected, it may be useful to add free T3 (FT3). Thyroid autoantibodies, particularly anti-TPO antibodies, help identify autoimmune thyroid disease.
3) What does an elevated TSH with a normal free T4 mean?
This combination usually defines subclinical hypothyroidism. In most cases, the finding should be confirmed by repeating laboratory tests after several weeks. The decision to treat depends on the TSH level, the presence of symptoms, positive anti-TPO antibodies, age, pregnancy or other risk factors.
4) When should levothyroxine treatment be started?
Treatment is generally recommended when TSH is ≥10 mU/L. At lower levels, the decision should take into account factors such as compatible symptoms, pregnancy or plans to conceive, infertility, ovulatory dysfunction, positive anti-TPO antibodies or a progressive rise in TSH over time.
5) What is the significance of positive anti-TPO antibodies?
Anti-TPO antibodies are markers of thyroid autoimmunity and are frequently present in Hashimoto thyroiditis. Their presence increases the risk of progression from subclinical to overt hypothyroidism and may influence the decision to start treatment.
6) What does a low TSH with a normal free T4 mean?
This pattern is compatible with subclinical hyperthyroidism. It should be confirmed by repeating the tests, since transient changes in TSH are relatively common. Persistent TSH suppression may warrant further investigation or specialist referral.
7) When should free T3 be measured?
Free T3 is particularly useful when TSH is low and free T4 is normal. In these circumstances, it may identify T3-predominant thyrotoxicosis, in which the only evident hormonal abnormality is an elevated free T3 level.
8) Which medicines can interfere with thyroid function?
The most relevant medicines include amiodarone, lithium and interferon. These medicines can cause thyroid dysfunction and therefore warrant periodic monitoring with TSH and free T4.
9) How should thyroid function be monitored during amiodarone treatment?
Assessment of TSH and free T4 before starting treatment is recommended, followed by periodic monitoring, usually every 6 months, because thyroid abnormalities are relatively common with this medicine.
10) When should central hypothyroidism be suspected?
Central hypothyroidism should be considered when there is a low free T4 associated with a low or inappropriately normal TSH. In these cases, overall pituitary function should be assessed and possible hypothalamic-pituitary disease investigated.
11) Should thyroid ultrasound be requested in every case?
No. Thyroid ultrasound is mainly indicated when structural disease is suspected, such as goitre, nodules or gland asymmetry. It should not be used routinely solely to investigate isolated laboratory abnormalities.
12) When should referral to Endocrinology be considered?
Referral should be considered in cases of persistent hyperthyroidism, suspected complex thyroid disease, central hypothyroidism, laboratory abnormalities that are difficult to interpret, pregnancy with significant thyroid disease or a need for specialist investigation.
