Hashimoto's disease is an autoimmune disordee that decreases the ability of the thyroid gland to produce T3 and T4. The student is modeling the reaction to increased TSH levels in an individual with untreated Hashimoto's disease compares to an individual without hashimoto's disease. Which of the following should be used as an independent variable?
Tsh level
TRH level
T3 level
T4 level