Depending on the region of the hypothalamus-pituitary-thyroid axis that is damaged, there are three types of hypothyroidism. In primary hypothyroidism, the failure occurs at the level of the thyroid gland, so that it stops producing T3 and T4 hormones. To try to compensate, the pituitary gland increases its secretion of TSH hormone. It is the most frequent form of hypothyroidism.
In the case of secondary hypothyroidism, the error takes place in the pituitary gland. For this reason, TSH hormone is no longer produced and, therefore, T3 and T4 levels also decrease.
Finally, tertiary hypothyroidism is the least common in the population. This type of hypothyroidism is due to alterations in the hypothalamus.