This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pituitary-Hypothalmic System and Thyroid, Subject name: Pharmacology
Manish, a patient of hypothyroidism was prescribed thyroxine. Which of the following is the most reliable guide for adjustment of thyroxine dose in him? 
Select and explain. 
A. Pulse rate, B. Body weight, C. Serum thyroxine level, D. Serum TSH level
D: Serum TSH level
Answer: D: Serum TSH level 
Explanation: * TSH levels are elevated in a hypothyroid patient (because feedback inhibition by thyroid hormones is not present). * Administration of thyroid hormones (T3 and T4) decreases TSH by contributing to feedback inhibition. * If TSH levels are less than normal, it signifies overeatment whereas high TSH level suggests inadequate treatment.