The primary rationale for screening asymptomatic non-pregnant patients is that the resulting treatment improves health outcomes when compared with patients who are not screened. There are no RCT or controlled observational studies in non-pregnant adults to assess the value of screening. Treating subclinical hypothyroidism (TSH ~4-10 IU/L and normal T3/T4) showed no benefits in any patient-oriented outcome such as mortality or cardiovascular disease, fatigue, weight, depression, cognitive function or quality of life.
TSH can vary up to 50% between tests and even up to 26% in one day in the same patient. The prevalence of subclinical hypothyroidism is 4-10% in the developing world.
Sources:
Allan M, Young J. Helping physicians fatigued by TSH Screening and Subclinical Hypothyroidism. Tools For Practice December 9, 2019.
Birtwhistle R. Morissett K, Dickinson J et al. Recommendation on screening adults for asymptomatic thyroid dysfunction in primary care. CMAJ November 18, 2019;191: (46) E1274-E1280. PMID: 31740537.
Best Practice Advocacy Centre New Zealand. Management of thyroid dysfunction in adults [Internet]. BPJ. 2010 Dec;(22):22-33 [cited 2014 Sep 25]. Available from: https://bpac.org.nz/BPJ/2010/December/thyroid.aspx.
U.S. Preventive Services Task Force. Screening for thyroid disease: recommendation statement. Ann Intern Med. 2004 Jan 20;140(2):125-7. PMID: 14734336.