Subclinical hypothyroidism is categorised as TSH levels above the range but with normal levels of thyroxine (T4) and triiodothyronine (T3).1,2 It is sometimes referred to as mild hypothyroidism or borderline hypothyroidism and is often linked with autoimmune thyroid disease (Hashimoto’s disease).2
Subclinical hypothyroidism is found in 8–10% of the population, is more common in women and increases with age.3 Subclinical hypothyroidism is more common in women of reproductive age. In some cases, but not all, it has been associated with fertility issues and an increased risk of adverse pregnancy.4
Whilst subclinical shows that there is a direct influence of thyroid hormones on the heart and blood vessels, it is still unclear if there are any connections between subclinical hypothyroidism and the increased risk of cardiovascular disease, neuropsychiatric and neuromuscular disorders.
One study concluded that subclinical hypothyroidism is associated with heart failure 5 but some are not clear.6 The Rotterdam Study showed that subclinical hypothyroidism is an independent risk factor for myocardial infarction and atherosclerosis in elderly women patients with an average TSH of 5.8 mIU/l, especially if they are positive for anti-TPO antibodies.7 Studies have shown some benefit in treating this issue with levothyroxine.8
Some studies have shown a connection between subclinical hypothyroidism and conditions such as depression and anxiety, but this was not the case across all studies.2
Subclinical hypothyroidism can be asymptomatic making diagnosis problematic; however, some patients may present with symptoms that would indicate overt hypothyroidism, the main ones being:9,10,11,12,13
Read Hypothyroid signs and symptoms for a fuller list.
Because subclinical hypothyroidism can be asymptomatic, a blood test is essential to confirm a diagnosis.
A thyroid function test can be arranged by your medical practitioner to check your TSH levels and ascertain if you are suffering from subclinical hypothyroidism or overt hypothyroidism.14
Other conditions may be investigated at the same time as your thyroid function test and it is important to check your vitamin and mineral status as low levels can cause issues connected to thyroid function.15
Levothyroxine is the standard medication prescribed for patients with an underactive thyroid.
However, if blood test results show that you have subclinical hypothyroidism but you are not experiencing any symptoms, then treatment may not be necessary. It may be preferable to have your hormone levels monitored on a regular basis and only consider treatment if levels change.16
We are aware that some women have been diagnosed with subclinical hypothyroidism at a health check appointment and even though they have no symptoms, have been prescribed levothyroxine which then makes them thyrotoxic – possibly because levothyroxine is not actually needed due to having no symptoms.
Treatment is often dependent on the individual practitioner’s preferences and can be largely intuitive rather than evidence-based. However, many patients with subclinical hypothyroidism do find levothyroxine beneficial.
Over the past few years, there have been discussions about changing the upper reference limit of TSH levels but as yet nothing has been confirmed.17
Subclinical hypothyroidism does not always lead to overt hypothyroidism especially if the cause is Hashimoto’s disease.
Dr A Toft, previous endocrinologist at the Royal Infirmary of Edinburgh, stated, "There is also the difficulty of interpreting a serum concentration of TSH in isolation. A concentration at or near the upper limit of the reference range, particularly if associated with a normal free T4, may indicate underlying autoimmune thyroid disease. A consensus exists for early treatment of such patients with thyroxine if anti-thyroid peroxidase antibodies are present in the serum, not because any immediate benefit may be expected but because the risk of overt thyroid failure in future years is high, and it makes sense to anticipate morbidity rather than risk loss to follow up." 18
If these are positive – indicative of underlying autoimmune thyroid disease – the patient should be considered to have the mildest form of hypothyroidism.
In the absence of symptoms, some would simply recommend annual thyroid function tests until serum TSH is over 10mU/l or symptoms such as tiredness and weight gain develop. But a more pragmatic approach is to recognise that the thyroid failure is likely to become worse and try to nip things in the bud rather than risk loss to follow-up." 18
Some practitioners believe that stress is a factor in Hashimoto’s disease and therefore reducing stress may reduce thyroid antibodies.19 There have also been some studies showing that avoiding gluten has reduced antibodies 20,21,22,23,24 and anecdotal evidence has shown that it is helpful.
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References
Date updated: 01.09.21 (V1.8)
Review date: 29.08.22
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