Which laboratory pattern is common in hypothyroidism?

• A. High T3 and T4, high TSH
• B. Low T3 and T4, high TSH
• C. Low T3 and T4, low TSH
• D. Normal T3/T4, normal TSH

Answer: (C)

When the thyroid isn’t producing enough hormone, the pituitary responds by trying to boost support—the pituitary pumps out more TSH to stimulate the thyroid. In the most common form of hypothyroidism, which is primary (thyroid gland failure), this leads to low thyroid hormones (T3 and T4) with a high TSH. So the classic lab picture is low T3/T4 with elevated TSH. If TSH is low or inappropriately normal while T3/T4 are low, that points to central or secondary hypothyroidism due to pituitary or hypothalamic problems, which is much less common. A pattern of high TSH with high T3/T4 would indicate hyperthyroidism, while normal levels in both typically indicate a euthyroid state.