A 27-year-old patient scheduled for foot surgery is evaluated for euthyroid status. Which laboratory test is most useful to determine euthyroid function?

A 27-year-old patient scheduled for foot surgery is evaluated for euthyroid status. Which laboratory test is most useful to determine euthyroid function?

• A. Total plasma thyroxine (T4)
• B. Total plasma triiodothyronine (T3)
• C. Thyroid-stimulating hormone (TSH)
• D. Resin triiodothyronine uptake

Answer: (C)

The main idea is that thyroid-stimulating hormone (TSH) is the most sensitive test for assessing overall thyroid function. The pituitary senses circulating thyroid hormones (T4 and T3) and adjusts TSH secretion accordingly. Small changes in thyroid hormone levels lead to larger, detectable changes in TSH, so it’s the best single indicator of whether the thyroid axis is functioning properly. In a euthyroid state, TSH sits in the normal reference range because pituitary output matches the thyroid’s hormone production. If TSH is elevated, it suggests hypothyroidism (often with a normal or low free T4), and if TSH is suppressed, it suggests hyperthyroidism (often with an elevated free T4). Because total T4 or T3 levels can be influenced by binding proteins and may remain normal in subclinical disease, they’re less reliable for screening euthyroid status. Resin uptake is an older test and not routinely used today. Free T4 can be helpful when TSH is abnormal, but for determining euthyroid status, TSH is the most informative initial test.

The main idea is that thyroid-stimulating hormone (TSH) is the most sensitive test for assessing overall thyroid function. The pituitary senses circulating thyroid hormones (T4 and T3) and adjusts TSH secretion accordingly. Small changes in thyroid hormone levels lead to larger, detectable changes in TSH, so it’s the best single indicator of whether the thyroid axis is functioning properly.

In a euthyroid state, TSH sits in the normal reference range because pituitary output matches the thyroid’s hormone production. If TSH is elevated, it suggests hypothyroidism (often with a normal or low free T4), and if TSH is suppressed, it suggests hyperthyroidism (often with an elevated free T4). Because total T4 or T3 levels can be influenced by binding proteins and may remain normal in subclinical disease, they’re less reliable for screening euthyroid status. Resin uptake is an older test and not routinely used today. Free T4 can be helpful when TSH is abnormal, but for determining euthyroid status, TSH is the most informative initial test.