Subclinical hypothyroidism

This is defined as an elevated serum TSH but a normal serum thyroxine concentration in an asymptomatic patient. The most common cause is chronic autoimmune thyroiditis (occurs in 3% of adults and 10% of postmenopausal women). Some patients with subclinical hypothyroidism will have common and nonspecific symptoms such as tiredness or weight gain.

The rate of progression from subclinical to overt hypothyroidism has been stated to be approximately 2 -5% per annum. Patients with a higher TSH value and those with positive for thyroid peroxidase antibody (TPOAb) are more likely to progress to overt hypothyroidism (2).

• in those patients with positive thyroid antibodies, it can be that up to 80% of patients will develop overt hypothyroidism over a four year period (3).

There is evidence to suggest that subclinical hypothyroidism may be an independent risk factor for coronary heart disease (4)

In people with subclinical hypothyroidism whohave positive TPO antibody titres (6)

• patients with subclinical hypothyroidism and positive TPO antibodies should be monitored more closely, with TSH checked every 6-12 months

Reference:

• (1) Association for Clinical Biochemistry (ACB), British Thyroid Association (BTA), British Thyroid Foundation (BTF) 2006. UK guidelines for the use of thyroid function tests
• (2) Todd CH. Management of thyroid disorders in primary care: challenges and controversies. Postgrad Med J. 2009;85(1010):655-9.
• (3) Prescriber 2002;13 (10): 50-68.
• (4) Walsh JP et al. Subclinical thyroid dysfunction as a risk factor for cardiovascular disease. Arch Intern Med 2005;165:2467-72.
• (5) Wood J. Thyroxine therapy. NEJM 1994; 331 (3): 174-80
• (6) Siskind SM et al. Investigating hypothyroidism.BMJ2021;373:n993http://dx.doi.org/10.1136/bmj.n993