Professional and Linguistic Assessments Board (PLAB) Practice Exam

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What type of thyroid dysfunction is often indicated by high T4, low TSH levels?

Hypothyroidism
Subacute thyroiditis
Hyperthyroidism
Euthyroid state

The correct answer is: Hyperthyroidism

Elevated levels of T4 (thyroxine) combined with low levels of TSH (thyroid-stimulating hormone) are indicative of hyperthyroidism. In this condition, the thyroid gland is overactive, producing more hormones than the body requires. This excess of T4 inhibits the release of TSH from the pituitary gland due to negative feedback mechanisms. When there is an abundance of thyroid hormones like T4 in the bloodstream, the pituitary recognizes that there is no need to produce more TSH, resulting in lowered levels of this hormone. In contrast, hypothyroidism is characterized by low T4 and high TSH, reflecting an underactive thyroid. Subacute thyroiditis might present with transient changes in hormone levels but is often associated with inflammation, and euthyroid state indicates normal thyroid hormone levels without dysfunction. Therefore, the combination of high T4 and low TSH is a classic representation of hyperthyroidism.