Which lab pattern is most consistent with primary hypothyroidism?

Which lab pattern is most consistent with primary hypothyroidism?

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

Answer: (C)

In primary hypothyroidism the thyroid gland isn’t making enough thyroid hormones (T3 and T4). When these hormones are low, the pituitary senses the drop and increases TSH to try to stimulate the thyroid. So the characteristic lab pattern is low T3 and T4 with a high TSH. The other patterns don’t fit because: - High T3/T4 with high TSH is unusual for a primary thyroid problem and would not reflect failing thyroid hormone production. - Normal T3/T4 with normal TSH indicates a euthyroid state. - Low T3 with high T4 but normal TSH is inconsistent—high T4 would typically suppress TSH, not leave it normal.

In primary hypothyroidism the thyroid gland isn’t making enough thyroid hormones (T3 and T4). When these hormones are low, the pituitary senses the drop and increases TSH to try to stimulate the thyroid. So the characteristic lab pattern is low T3 and T4 with a high TSH.

The other patterns don’t fit because:

• High T3/T4 with high TSH is unusual for a primary thyroid problem and would not reflect failing thyroid hormone production.

• Normal T3/T4 with normal TSH indicates a euthyroid state.

• Low T3 with high T4 but normal TSH is inconsistent—high T4 would typically suppress TSH, not leave it normal.