23rd May 2025

Subclinical hypothyroidism

This condition refers to a clinical picture of raised TSH in the presence of normal free thyroid hormone concentrations in an asymptomatic patient. It is most commonly caused by chronic autoimmune thyroiditis. It occurs more commonly in postmenopausal women. 

Clinical picture and investigations

• Many patients will be asymptomatic, and subclinical hypothyroidism will be picked up on blood tests done for other reasons.
• Some patients may have mild nonspecific symptoms such as tiredness or weight gain. It can progress to overt hypothyroidism with a rate 2-5% per annum. There is increased incidence of cardiovascular risk which is greater if <65 years old. 
• As mentioned above, TSH will be raised but thyroid hormones are within normal. Patients who show +ve anti-TPO antibodies (TPOABs) are more likely to develop overt hypothyroidism. Thus, monitoring for TSH is required for them for 6-12 months. If TPOAbs are -ve, then TSH monitoring is advised every 3-5 years.

Management

There is some controversy with regards to treatment of subclinical hypothyroidism. Some authorities prefer to start treatment before progression to overt hypothyroidism. NICE guidelines suggest:

• Measuring TPOAbs for adults with TSH above the reference range.  
• Considering treatment with levothyroxine for adults when TSH is ≥ 10mIU/L on 2 separate occasions of 3 months apart. 
• 6-month trial of levothyroxine for adults <65 with TSH findings above reference range but below 10mIU/L on 2 separate occasions (3-months apart) and symptoms of hypothyroidism. NICE advises to re-adjust dose accordingly if symptoms do not improve. If TSH falls below 10mIU/L, then levothyroxine is advised to stop but monitoring continues.

In case of subclinical hypothyroidism due to non-thyroidal illness or transient thyroiditis from which the patient is recovering, it is advised not to start treatment on the basis of a single raised TSH level with negative TPOAbs.