5. Low TSH induced hypothyroidism
Secondary and Tertiary hypothyroidism
Primary hypothyroidism means that there is a problem with the thyroid gland itself. It is unable to make enough thyroid hormone.
The term secondary hypothyroidism refers to a disease of the pituitary gland. If the pituitary does not making enough TSH to stimulate the thyroid gland then the thyroid is not going to make enough thyroid hormone.
Tertiary hypothyroidism refers to a disorder of another part of the brain called the hypothalamus. The hypothalamus makes Thyroid Releasing Hormone (TRH) is what tells the pituitary how much TSH to make.
Secondary and tertiary hypothyroidism can be caused by tumors in either the pituitary or hypothalamus. These are very rare conditions, and the treatment is beyond the scope of this guide.
More common are functional problems that interfere in the pituitary's production of TSH.
Labs which reflect this type of thyroid hormone are:
- Low normal TSH (broadly speaking, less than 1.5).
- T3 low, or bottom end of reference range.
- T4 low or bottom end of reference range.
- Reverse T3 may be elevated (indicate issue with thyroid hormone conversion. 
This pattern of hypothyroidism often does not respond well to thyroid hormone medications. For more information on, go to the section on Euthyroid Sick Syndrome and Subclinical Hypothyroidism.