The causes of hypothyroidism are broadly classified as primary, secondary, and tertiary, as summarized in Table 3.
Primary hypothyroidism relates to defects/disease of the thyroid itself and are further delineated below. The most common cause is an autoimmune mechanism, where the body perceives healthy and normally functioning thyroidal tissue or products as foreign and attacks them.
Secondary hypothyroidism is attributed to insufficient stimulation of the thyroid, most commonly due to a lack of thyroid-stimulating hormone (TSH) production from the pituitary gland.
Tertiary hypothyroidism is a rare form of hypothyroidism that occurs when there is insufficient thyrotropin-releasing hormone (TRH) production from the hypothalamus.
Table 3. Types of Hypothyroidism.| Type | Origin | Cause |
| Primary | Thyroid | Autoimmune interference with productionInsufficient supplies to make hormonesDefective or blocked receptorsFaulty processing of hormone synthesis |
| Secondary | Pituitary | Insufficient TSH production from the pituitary Common |
| Tertiary | Hypothalamus | Insufficient TRH production from the hypothalamus Rare |
