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The Thyroid Gland Quiz

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In the initial blood test assessment of suspected thyroid disease, what does a raised thyroid-stimulating hormone (TSH) level most strongly indicate?

A A TSH-secreting pituitary adenoma, with free T4 needed to show that thyroid hormones are also elevated
B Primary hyperthyroidism, with free T4 needed to confirm thyrotoxicosis
C Primary hypothyroidism, with free T4 needed to distinguish overt from subclinical disease ✓
D Normal thyroid function with physiological variation, with free T4 needed to confirm the normal hormone concentration
E Acute thyroid inflammation causing transient hormone release, with free T4 needed to distinguish the thyrotoxic phase from the later hypothyroid phase

The answer is C: Primary hypothyroidism, with free T4 needed to distinguish overt from subclinical disease

Explanation

Why C is correct

See Thyroid Hormone Regulation. Option C is correct because thyroid hormone secretion is controlled by negative feedback: when thyroid hormone falls, the pituitary releases more TSH to push the thyroid harder. A raised TSH therefore points to a thyroid that cannot make enough hormone, which is primary hypothyroidism (the problem lies in the thyroid itself rather than in the pituitary). Because TSH is the first test, the laboratory measures free T4 (the unbound, active form of thyroxine) in the same sample when TSH is raised. A raised TSH with a low free T4 suggests primary hypothyroidism, and a raised TSH with a normal free T4 suggests subclinical hypothyroidism (a raised TSH but normal thyroid hormone levels). In non-pregnant people the tests are repeated after 3 to 6 months before the diagnosis is confirmed, because a raised TSH is sometimes transient. In the UK, which has enough iodine in the diet, the commonest cause is autoimmune thyroiditis, an immune attack on the gland. A memory aid: TSH and thyroid hormone move in opposite directions.

Negative feedback: when thyroid hormone falls, the pituitary releases more TSH.
Raised TSH: it points to primary hypothyroidism, where the thyroid itself cannot make enough hormone.
Free T4: a low level suggests primary hypothyroidism, and a normal level suggests subclinical hypothyroidism.

Why the other options are incorrect

A. A TSH-secreting pituitary adenoma, with free T4 needed to show that thyroid hormones are also elevated: A TSH-secreting pituitary adenoma is a rare cause of a raised TSH, whereas primary hypothyroidism is the commonest and strongest indication. In a TSHoma, free T4 is also elevated rather than low, which is a different pattern.
B. Primary hyperthyroidism, with free T4 needed to confirm thyrotoxicosis: In primary hyperthyroidism the high thyroid hormone level suppresses pituitary TSH, so the initial result would be a low TSH, not a raised one. Free T4 is used to confirm thyrotoxicosis when TSH is suppressed, but this does not fit the raised TSH described.
D. Normal thyroid function with physiological variation, with free T4 needed to confirm the normal hormone concentration: A raised TSH is by definition outside the normal range, so normal thyroid function cannot be the strongest indication. Transient TSH rises occur and are repeated, but they do not make the raised result normal.
E. Acute thyroid inflammation causing transient hormone release, with free T4 needed to distinguish the thyrotoxic phase from the later hypothyroid phase: Thyroid inflammation initially releases stored hormone, which raises free T4 and suppresses TSH; a raised TSH does not fit this phase. Free T4 is used to track the phases, but it is not the condition indicated by the raised TSH.

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More questions from this set. Start the quiz to answer them and see each explanation.

1
Which statement best describes the classical genomic action of thyroid hormone?
2
Which step in thyroid hormone synthesis is directly catalysed by thyroid peroxidase?

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