Which laboratory pattern is typical of primary hypothyroidism?

Which laboratory pattern is typical of primary hypothyroidism?

• A. High TSH and Low T4
• B. Low TSH and High T4
• C. High TSH and High T4
• D. Low TSH and Low T4

Answer: (A)

Understanding how the pituitary and thyroid glands communicate clarifies this pattern. In primary hypothyroidism the thyroid gland itself is underproductive, so thyroid hormone levels fall. The pituitary senses the low thyroid hormone and responds by increasing TSH to try to stimulate the thyroid. Since the thyroid is failing, T4 remains low despite the high TSH. So the typical laboratory pattern is elevated TSH with low T4 (often the free T4 is measured to reflect the active hormone). This contrasts with secondary hypothyroidism, where the pituitary isn’t producing enough TSH and T4 is low with a low or inappropriately normal TSH, and with hyperthyroidism, where TSH is suppressed and T4 is high.

Understanding how the pituitary and thyroid glands communicate clarifies this pattern. In primary hypothyroidism the thyroid gland itself is underproductive, so thyroid hormone levels fall. The pituitary senses the low thyroid hormone and responds by increasing TSH to try to stimulate the thyroid. Since the thyroid is failing, T4 remains low despite the high TSH. So the typical laboratory pattern is elevated TSH with low T4 (often the free T4 is measured to reflect the active hormone). This contrasts with secondary hypothyroidism, where the pituitary isn’t producing enough TSH and T4 is low with a low or inappropriately normal TSH, and with hyperthyroidism, where TSH is suppressed and T4 is high.