Medicines for Subclinical hypothyroidism

What it is

Subclinical hypothyroidism is a mild or early form of an underactive thyroid, defined by the pattern on thyroid blood tests: the level of TSH (the pituitary hormone that stimulates the thyroid) is raised, but the level of the main thyroid hormone (thyroxine, T4) is still within the normal range. This is often described as "borderline" or "subclinical" because the thyroid is having to work a little harder (shown by the raised TSH) but is still producing normal amounts of thyroid hormone, so the person may have no symptoms, or only mild, non-specific ones (such as tiredness). It is common, particularly in older people and women, and is often found on a routine or incidental blood test. Its significance is variable: in some people it remains stable or even returns to normal, while in others it progresses over time to a fully underactive thyroid (overt hypothyroidism) needing treatment. The chance of progression is higher when the TSH is more raised, and when certain thyroid antibodies are present. Because it is a borderline state, decisions about whether to treat it or simply monitor it are individualised, based on factors such as how high the TSH is, whether there are symptoms, the person's age, and other circumstances (such as pregnancy or trying to conceive, where treatment is more likely to be advised).

How it is treated

Management of subclinical hypothyroidism is individualised, and often involves monitoring rather than immediate treatment, guided by how borderline the results are and the person's circumstances. Because a single borderline result can be temporary (for example after an illness), the thyroid tests are usually repeated after a period to confirm the pattern, and thyroid antibodies may be checked to help judge the likelihood of progression. Where the TSH is only mildly raised and the person has no or minimal symptoms, the usual approach is watchful monitoring with periodic thyroid blood tests, to detect if it progresses to a fully underactive thyroid (at which point treatment is started). Treatment with thyroid hormone replacement (levothyroxine) is more likely to be recommended in certain situations — for example when the TSH is more substantially raised, when there are clear symptoms that might respond, in younger people, and particularly in women who are pregnant or planning pregnancy (where adequate thyroid function is important for the pregnancy and baby, so treatment is more readily given). Where treatment is started, the dose is adjusted and monitored. The decision is made with the individual, weighing the likely benefits. The reassuring message is that subclinical hypothyroidism is a common, borderline finding that often just needs monitoring, that it may remain stable or progress, and that treatment is considered where it is more likely to help — with particular attention in pregnancy.