A lab shows high TSH with low T4. Which statement best describes this finding?

A lab shows high TSH with low T4. Which statement best describes this finding?

• A. The thyroid is overproducing hormone.
• B. The pituitary is not communicating properly.
• C. The pituitary is signaling the thyroid to produce more, but the thyroid is not responding adequately.
• D. The patient needs no further evaluation.

Answer: (C)

High TSH with low T4 shows the pituitary is trying to drive the thyroid to make more hormone, but the thyroid isn’t responding adequately. This is primary hypothyroidism—the thyroid gland is underactive, and the pituitary responds by increasing TSH. If the problem were pituitary failure, TSH would be low or inappropriately normal despite low T4. If the thyroid were overactive, T4 would be high and TSH would be suppressed. This pattern signals the need for further evaluation and treatment planning.

High TSH with low T4 shows the pituitary is trying to drive the thyroid to make more hormone, but the thyroid isn’t responding adequately. This is primary hypothyroidism—the thyroid gland is underactive, and the pituitary responds by increasing TSH. If the problem were pituitary failure, TSH would be low or inappropriately normal despite low T4. If the thyroid were overactive, T4 would be high and TSH would be suppressed. This pattern signals the need for further evaluation and treatment planning.