甲狀腺激素在維持整體健康佔有極重要的角色，甲狀腺功能低下的影響包括血脂、血壓、體重及身體質量指數的上升、代謝症候群發生率的增加，內皮細胞功能異常，這些風險因子皆已被證實：在罹患甲狀腺功能低下的患者，會導致心血管疾病與死亡率增加，因此有必要投與甲狀腺素製劑治療。然而，因著甲狀腺功能檢測日漸普及，臨床上遇到亞臨床甲狀腺功能低下( 定義為：游離四碘甲狀腺素數值正常，但是甲狀腺刺激素高於正常參考值，通常為 > 4.5 uIU/mL) 的機率亦隨之增加，這個疾病雖然亦有前述各種心血管疾病風險因子的出現，但是目前並無確切證據顯示會導致心血管疾病發生率與相關死亡率的增加，且投予甲狀腺素製劑治療的臨床實驗亦未顯示在65 歲以上的族群可獲致明顯療效，因此是否需積極以藥物補充治療的議題仍待大型臨床試驗釐清。依據目前的治療指引建議：若是甲狀腺刺激素> 10 uIU/mL，因已有臨床追蹤觀察研究顯示會導致心臟衰竭風險升高，應該投與甲狀腺素製劑治療，起始劑量宜低( 每日12.5-25μg，每四至八周逐漸增加12.5-25 μg)，大部分的患者以每日25-75 μg 的總量可達到明顯下降甲狀腺刺激素至正常範圍。甲狀腺刺激素的治療目標值因年齡而異：60歲以下：1-2.5 uIU/mL, 60-70 歲：3-4 uIU/mL，70 歲以上的族群則可放寬至4-6 uIU/mL。定期檢測是必要的，要避免因劑量過高導致的醫源性甲狀腺功能亢進。

With the wide application of thyroid function tests performed in current clinical medicine, including field of preventive medicine, subclinical hypothyroidism has become an increasingly recognized clinical entity in daily practices encountered by physicians in general practice and subspecialties, including endocrinologists and geriatricians. The diagnosis may not be difficult since measured levels of thyroid-stimulating hormone (TSH), by definition higher than the upper normal limit of the laboratory references, and free thyroxine (fT4) within the normal reference range can provide enough biochemical evidence for a tentative diagnosis, although repeated tests are often required for confirmative purpose due to the largely unpredictable natural course of this endocrine disease. The presence of autoimmune antibodies (especially thyroid-peroxidase antibodies) significantly increases the risk of future development into overt hypothyroidism. Beyond that, challenges in clinical scenario, especially in the elderly population, may lie in the decision of providing optimal management by administration or not of levothyroxine supplement therapy, even when a diagnosis has been given. Indications for treating subclinical hypothyroidism include a desired improvement in symptoms, prevention of adverse events associated (especially the cardiovascular disorders), as well as prevention of overt hypothyroidism. Current guidelines from academic societies recommend that, in those with TSH levels ≥ 10 uIU/mL, small doses (eg, 25-75 μg per day) of levothyroxine usually suffice to restore normal serum thyrotropin levels in the majority of non-pregnant patients. The targets of TSH levels are recommended to set by age groups: for younger patient (< 60 years): 1-2.5 uIU/mL, whereas enlarged to 3-4 uIU/mL in patients between 60-70 years and 4-6 uIU/mL when older than 70 years. However, these potential benefits of levothyroxine supplementation should be weighed against the risks of reducing thyrotropin values below the reference range and potentially causing iatrogenic subclinical or overt hyperthyroidism.