Supplements containing biotin should be withheld for several days before assessing thyroid function.
An exact reference range for TSH is not widely agreed upon—although most laboratories regard 4.0 to 5.0 mIU/L as the high-end cutoff for normal. Also, “normal” TSH levels appear to differ by age. Accordingly, some experts have recommended an age-based reference range for TSH levels,15 although this is not implemented widely by laboratories. A TSH level of 6.0 mIU/L (or even higher) may be more appropriate for adults older than 65 years.1
Biotin supplementation has been shown to cause spurious thyroid testing results (TSH, T3, T4) depending on the type of assay used. Therefore, supplements containing biotin should be withheld for several days before assessing thyroid function.16Patients with SCH are often categorized as having TSH levels between 4.5 and 10 mIU/L (around 90% of patients) or levels ≥ 10 mIU/L.8,17 If followed for 5 years, approximately 60% of patients with SCH and TSH levels between 4 and 10 mIU/L will normalize without intervention.18 Normalization is less common in patients with a TSH level greater than 10 mIU/L.18
The risk for progressionto overt hypothyroidism also appears to be higher for those with certain risk factors. These include higher baseline TSH levels, presence of thyroid peroxidase antibodies (TPOAbs), or history of neck irradiation or radioactive iodine uptake.1 Other risk factors for eventual thyroid dysfunction include female sex, older age, goiter, and high iodine intake.13
Evidence for treatment varies
Guidelines for the treatment of SCH (TABLE 18,14,19,20) are founded on the condition’s risk for progression to overt hypothyroidism and its association with health consequences such as cardiovascular disease. Guidelines of the American Thyroid Association (ATA) and European Thyroid Association (ETA), and those of the United Kingdom–based National Institute for Health and Care Excellence (NICE), prioritize treatment for individuals with a TSH level > 10 mIU/La and for those with TSH values < 10 mIU/L but still elevated and apparent symptoms of hypothyroidism.14,19,20 The strength of evidence behind this guidance is challenged by a lack of data from prospective randomized controlled trials (RCTs) demonstrating health benefits following treatment of SCH. The British Medical Journal (BMJ) Guideline cites this lack of evidence and recommends against treating SCH at any TSH level, regardless of symptoms.8
There are few large RCTs of treatment outcomes for SCH. A 2017 RCT (the Thyroid Hormone Replacement for Untreated Older Adults with Subclinical Hypothyroidism, or TRUST, trial) of 737 adults older than 65 years with SCH evaluated the ability of levothyroxine to normalize TSH values compared with placebo. At 1 year, there was no difference in hypothyroid symptoms or tiredness scale scores with levothyroxine treatment compared with placebo.21 This finding was consistent even in the subgroup with a higher baseline symptom burden.22
