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Subclinical Hypothyroidism

What is subclinical hypothyroidism?

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

What are the symptoms of subclinical hypothyroidism?

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

  • fatigue/lethargy
  • weight gain, even though you eat less
  • cold intolerance
  • muscle weakness, arthralgia, myalgia
  • constipation
  • menstrual problems
  • depression
  • impaired concentration and memory
  • hair loss body, scalp and eyebrows
  • hoarse voice/deepening of voice
  • dry skin and hair
  • goitre, difficulty swallowing
  • delayed relaxation of deep tendon reflexes
  • puffy face
  • loss of libido
  • fertility problems
  • raised cholesterol

Read Hypothyroid signs and symptoms for a fuller list.

Testing for subclinical hypothyroidism

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

Treatment for subclinical hypothyroidism

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

Prognosis for subclinical hypothyroidism

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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  1. Thyroid hormones treatment for subclinical hypothyroidism: a clinical practice guideline
    G E Beckering et al
    BMJ 2019;365:l2006
  2. Subclinical Hypothyroidism – Whether and When To Start Treatment?
    Milena Cojić and Ljiljana Cvejanov-Kezunović
    Open Access Maced J Med Sci. 2017 Dec 15; 5(7): 1042–1046.
  3. Subclinical thyroid disease
    Anukul Garg, Mark P.J. Vanderpump
    British Medical Bulletin, Volume 107, Issue 1, September 2013, Pages 101–116,
  4. Subclinical hypothyroidism in women….
  5. Association of…
  6. Hypothyroidism and the Heart
    Maja Udovcic, M.D et al
    Methodist Debakey Cardiovasc J. 2017 Apr-Jun; 13(2): 55–59.
  7. Subclinical Hypothyroidism Is an Independent Risk Factor for Atherosclerosis and Myocardial Infarction in Elderly Women: The Rotterdam Study
  8. The Beneficial Effect of L-Thyroxine on Cardiovascular Risk Factors, Endothelial Function, and Quality of Life in Subclinical Hypothyroidism: Randomized, Crossover Trial
  9. British Thyroid Foundation
  10. Underactive thyroid (hypothyroidism) – Symptoms
  11. Goitre
  12. You and your Hormones – Goitre
    Society for Endocrinology
  13. A Classic Sign of Hypothyroidism
    S Cyriac et al
    CMAJ, 2008 Aug 12; 179(4): 387
  14. Underactive thyroid (hypothyroidism) – Treatment
  15. Vitamin D and Thyroid disease to D or not to D
  16. Underactive thyroid: Deciding whether or not to treat subclinical hypothyroidism
  17. Subclinical Hypothyroidism: Considerations in defining the lower limit of the Thyrotropin reference interval.
  18. Thyroid Function Tests and Hypothyroidism – Measurement of serum TSH alone may not always reflect thyroid status
  19. The connection between stress and Hashimotos
    Dr Teresa C Hill
  20. The Effect of Gluten-Free Diet on Thyroid Autoimmunity in Drug-Naïve Women with Hashimoto’s Thyroiditis: A Pilot Study.
    Krysiak R1, Szkróbka W1, Okopień B1.
    Exp Clin Endocrinol Diabetes. 2019 Jul;127(7):417-422
  21. The Gluten-Thyroid Connection
    Chris Kessler
  22. The presence of antigiliadin antibodies in ….
  23. Autoimmune thyroid diseases and coeliac disease
  24. Clinical and subclinical autoimmune thyroid disease in adult celiac disease.

Date updated: 01.09.21 (V1.8)
Review date: 29.08.22

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