Definition and symptoms
- Subclinical hypothyroidism is a biochemical diagnosis defined as an elevated serum TSH concentration with a serum FT4 concentration within the normal range.
- Typical symptoms of hypothyroidism include fatigue, weight gain, constipation and dry skin. However, these symptoms are extremely prevalent and non-specific for hypothyroidism, and can often be attributed to lifestyle factors.9 These symptoms are non-diagnostic, especially in early disease presentation.10
- Many factors influence the TSH, including:6,11
- concomitant disease
- concurrent illness
- iodine status
- circadian rhythms
- heterophilic antibodies
In addition, medications and supplements can impact the TSH, such as iodine excess (for example, contrast, kelp tablets), lithium, amiodarone, interferon alfa, interleukin-2, tyrosine kinase inhibitors and immune checkpoint inhibitors.12
Monitoring and management
- In many cases, asymptomatic subclinical hypothyroidism with a TSH between 4 and 10 mIU/L can be monitored and does not need to be treated.
- Patients with subclinical hypothyroidism and positive TPOAbs are more likely to develop overt hypothyroidism, but can be managed with monitoring if they are asymptomatic.4
- There is no indication to measure triiodothyronine (FT3) or reverse triiodothyronine (reverse T3) in the routine assessment and management of hypothyroidism.13
- Research14 indicates that a patient’s request for additional thyroid testing and treatment can act as a barrier to appropriate management.
- Thyroid ultrasound is indicated in the assessment of palpable goitre and thyroid nodules, but it is not part of the routine assessment of hypothyroidism.3,13 Overuse of ultrasound can identify the presence of clinically unimportant thyroid nodules and can lead to the overdiagnosis of thyroid cancer.3
- Some articles in the lay press have suggested that traditional thyroid function testing is not reliable, and this has contributed to a degree of medical mistrust.9
- Individual treatment should be based on previous radioactive iodine treatment, surgery, elevated thyroid autoantibodies or elevated cardiovascular risk.4,10
- Asymptomatic subclinical hypothyroidism >10 mIU/L is associated with an increased risk of developing symptoms and cardiovascular events, and therefore treatment should be considered.5,15
Treatment with levothyroxine
- Although multiple studies show the association between subclinical hypothyroidism and cardiovascular disease, there is no clear evidence that treatment with levothyroxine to reduce cardiovascular risk is beneficial.5,16
- If symptoms do not improve after starting levothyroxine, re-measure TSH. If the level remains raised, adjust the dose. If TSH is within the reference range and symptoms persist, consider ceasing treatment and monitoring, and consider the possibility of an alternative diagnosis.4
Treatment with liothyronine
There is no evidence to support the use of liothyronine (alone or in combination with levothyroxine) in patients with subclinical hypothyroidism.5
Outcomes of correction of TSH
- Correction of TSH to within the reference range does not always bring thyroid and other biomarkers (especially lipids) into range, and if there are only small deviations in TSH, it rarely changes biomarkers or alleviates patient symptoms.6
- Most patients with subclinical hypothyroidism do not benefit from treatment unless TSH >10 mIU/L and/or TPOAbs is elevated.10
As there is no evidence that population screening is beneficial,10 investigation should be limited to patients presenting with clinical indications.