A laboratory result shows high TSH and low T4. Which explanation is correct?

A laboratory result shows high TSH and low T4. Which explanation is correct?

• A. It means inconsistency in your thyroid tests and you will need more testing.
• B. I am sorry. You will have to ask your physician about your laboratory results. We are not allowed to discuss them.
• C. The TSH is sending a message to your thyroid gland to increase production, but your thyroid isn't producing enough hormone.
• D. That means you will have to go on hormone therapy for the rest of your life.

Answer: (C)

When TSH is high and T4 is low, it shows a problem at the thyroid gland itself rather than in the pituitary. The pituitary increases TSH when thyroid hormone is low in an attempt to stimulate the thyroid to produce more T4. If T4 remains low despite high TSH, the thyroid isn’t responding adequately—this is primary hypothyroidism. So the best explanation is that the TSH is signaling the thyroid to increase production, but the thyroid isn’t producing enough hormone. The other interpretations aren’t as accurate for describing this pattern of thyroid function.

When TSH is high and T4 is low, it shows a problem at the thyroid gland itself rather than in the pituitary. The pituitary increases TSH when thyroid hormone is low in an attempt to stimulate the thyroid to produce more T4. If T4 remains low despite high TSH, the thyroid isn’t responding adequately—this is primary hypothyroidism. So the best explanation is that the TSH is signaling the thyroid to increase production, but the thyroid isn’t producing enough hormone. The other interpretations aren’t as accurate for describing this pattern of thyroid function.