亞臨床甲狀腺低能症的定義是血清中促甲狀腺素（thyroid stimulating hormone, TSH）濃
度偏高，但血清甲狀腺素濃度仍在正常範圍內者。其病因與甲狀腺低能症相同，通常病人是無症狀或僅有輕微非特異性的症狀。診斷上主要是以實驗室檢驗為準，但需排除其他會造成TSH升高的因素，如euthyroid sick syndrome恢復期或腎上腺功能不足等。有時需要重覆甲狀腺功能檢驗以確定診斷。目前為止尚沒有足夠的證據能證明甲狀腺素補充在這類病人的好處。以下情況則可以考慮給予甲狀腺素補充治療：TSH濃度高於10 mIU/L或TSH偏高但低於10 mIU/L且有甲狀腺自體抗體者。懷孕婦女若有亞臨床甲狀腺低能症也建議給予甲狀腺素補充。若具有亞臨床甲狀腺低能症危險因子之婦女懷孕或計畫懷孕者，應先作甲狀腺功能檢驗。

Subclinical hypothyroidism is defined as elevated serum thyroid
stimulating hormone (TSH) while normal free T4 (FT4) levels. The etiology of subclinical hypothyroidism is the same as that of overt hypothyroidism.
Patients with subclinical hypothyroidism may be asymptomatic or have only mild non-specific symptoms. The diagnosis is based on laboratory results but other causes of TSH elevation such as recovery form nonthryoid
illness or adrenal insufficiency should be rule-out before making the diagnosis. Repeating the laboratory tests may be necessary to confirm the diagnosis. Treatment with thyroxine may be considered in those who has
TSH level higher than 10 IU/mL, or has elevated TSH level but lower than 10 IU/mL and anti-thyroid peroxidase antibody positive. In pregnant women with subclinical hypothyroidism, thyroxine supplement is recommended.
Thyroid function tests may be considered in women with risk factors for subclinical hypothyroidism when pregnant or pregnancy is intended.