This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Medicine
Which of the following is found in primary hypothyroidism- 
Select and explain. 
A. T3 decrease, T4 decrease, TSH increase, B. T3 decrease, T4 increase, TSH decrease, C. T3 normal, T4 normal, TSH increase, D. T3 decrease, T4 decrease, TSH normal
A: T3 decrease, T4 decrease, TSH increase
Answer: A: T3 decrease, T4 decrease, TSH increase 
Explanation: In the vast majority of cases, hypothyroidism results from an intrinsic disorder of the thyroid gland (primary hypothyroidism). In this situation, serum T4 is low and TSH is elevated, usually in excess of 20 mU/L. Measurements of serum T3 are unhelpful since they do not discriminate reliably between euthyroidism and hypothyroidism. Secondary hypothyroidism is rare and is caused by failure of TSH secretion in an individual with hypothalamic or anterior pituitary disease. Other non-specific abnormalities are shown in Box 20.9. In severe, prolonged hypothyroidism, the electrocardiogram (ECG) classically demonstrates sinus bradycardia with low-voltage complexes and ST segment and T-wave abnormalities. Measurement of thyroid peroxidase antibodies is helpful but fuher investigations are rarely required. DAVIDSONS PRINCIPLES AND PRACTICE OF MEDICINE 22ND EDITION PAGE NO-743