Primary hypothyroidism means a disease of the thyroid gland itself is causing hypothyroidism.
The term secondary hypothyroidism refers to a a disease of the pituitary gland which prevents it from making TSH. If the pituitary can't make enough TSH, then the thyroid won't make enough thyroid hormone, even if there is nothing wrong with the thyroid gland itself.
Tertiary hypothyroidism refers to a disorder of another part of the brain called the hypothalamus. The hypothalamus makes several hormones which tell the pituitary gland what to do. One of these hormones, thyroid releasing hormone (TRH) is what tells the pituitary how much TSH to make.
Secondary and tertiary hypothyroidism are often caused by tumors in either the pituitary or hypothalamus. These are very rare conditions, and the treatment is beyond the scope of this guide.
However, there are some other factors that could reduce pituitary production of TSH. Labs that would be seen in such a case are low T3, with normal TSH and T4. Normally, if T3 is low, then TSH should be elevated. This may be seen when patients have increase conversion of T4 into the inactive revere T3, instead of the active T3. 
We will go into more detail of this functional (functional meaning decreased organ function, yet no diseased tissue) cause of hypothyroidism symptoms starting in section 11.
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