From JAMA Network:
Subclinical hypothyroidism is common [up to 10% of adults] and most individuals can be observed without treatment. Treatment might be indicated for patients with subclinical hypothyroidism and serum thyrotropin [TSH] levels of 10 mU/L or higher or for young and middle-aged individuals with subclinical hypothyroidism and symptoms consistent with mild hypothyroidism.
Source: Subclinical Hypothyroidism: A Review | Cardiology | JAMA | JAMA Network