What is the best screening test for hyperthyroidism in a pregnant adolescent?

The serum thyroid stimulating hormone (TSH) level is the best screening test for hyperthyroidism, especially in a pregnant adolescent. During pregnancy, changes in the hormonal environment can affect thyroid function, and TSH is a key hormone that regulates thyroid activity. In a healthy individual, TSH levels are inversely related to thyroid hormone levels—meaning that if TSH is low, it indicates that the thyroid is producing excess hormones, which is characteristic of hyperthyroidism.

Measuring TSH is particularly useful because it is sensitive to even small shifts in thyroid function and can reliably indicate thyroid status. If the TSH level is suppressed, further testing, such as free T4 or total T4, may be warranted to confirm hyperthyroidism.

Other tests, such as serum thyroid-binding globulin levels and serum total thyroxine levels, do provide some information but are not as specific for initial screening. Dietary iodine levels do not directly assess thyroid function and are more indicative of dietary intake rather than the physiological functioning of the thyroid gland. Thus, the most effective way to screen for hyperthyroidism in a pregnant adolescent is by evaluating the serum TSH level.