甲状腺机能低下症，又称作甲状腺功能减退（简称甲减），是一种常见的由甲状腺分泌的甲状腺素不足导致内分泌疾病。甲减可以引起多种症状，如疲劳，御寒能力低下，体重增加等。甲减可以造成儿童的身体和智力的发育迟缓，在某些情况下发展成侏儒症。甲减可以通过检测血液中促甲状腺素（TSH）的水平和甲状腺激素T₄的水平来诊断。

在世界范围内，食物中缺少碘元素是造成甲减的最主要原因。而在一些食物中并不缺碘的国家，甲减主要是由于一种叫做桥本氏甲状腺炎自身免疫疾病造成的。还有少量的甲减是由于接受过放射性碘元素进行治疗，甲状腺损伤，脑下垂体前叶损伤，或服用某些药物，先天性甲状腺机能低下，或甲状腺手术创伤引起的。

Hypothyroidism, also called underactive thyroid or low thyroid, is a common disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as poor ability to tolerate cold, a feeling of tiredness, constipation, depression, and weight gain. Occasionally there may be swelling of the front part of the neck due to goiter. Untreated hypothyroidism during pregnancy can lead to delays in growth and intellectual development in the baby, which is called cretinism.