A 9-year-old boy is brought to the pediatrician by his parents for evaluation of fatigue, … — USMLE Step 2 CK

A 9-year-old boy is brought to the pediatrician by his parents for evaluation of fatigue, constipation, and poor growth. The parents report that he has been feeling cold even in warm weather and has not outgrown any of his clothes over the past year. Review of his growth chart shows a decline in height from the 50th percentile to the 10th percentile. Physical examination reveals dry skin and delayed relaxation of the ankle reflexes. Laboratory evaluation shows a free T4 level of $0.5 \text{ ng/dL}$ (normal: $0.9-1.7 \text{ ng/dL}$ ) and a thyroid-stimulating hormone (TSH) level of $2.1 \text{ }\mu\text{U/mL}$ (normal: $0.5-4.5 \text{ }\mu\text{U/mL}$ ). Which of the following is the most likely diagnosis?

Central hypothyroidismAnswer
B
Hashimoto thyroiditis
C
Constitutional delay of growth and puberty
D
Thyroid hormone resistance
E
Graves disease

Answer

Central hypothyroidism

The diagnosis of central (secondary or tertiary) hypothyroidism is characterized by a low free thyroxine (T4) level accompanied by a thyroid-stimulating hormone (TSH) level that is inappropriately normal, low, or slightly elevated. In a normally functioning hypothalamic-pituitary-thyroid axis, a low T4 should trigger a significantly elevated TSH. The lack of TSH elevation in this symptomatic patient indicates a failure at the level of the pituitary gland or hypothalamus.

Step-by-Step Solution

Analyze the patient's clinical presentation.

Symptoms include fatigue, cold intolerance, constipation, poor linear growth, dry skin, and delayed deep tendon reflexes.

These findings are classic for a hypothyroid state, prompting the need to review thyroid function tests.

Evaluate the free T4 level.

The free T4 is

0.5 \text{ ng/dL}

, which is well below the normal reference range.

A distinctly low free T4 confirms the diagnosis of clinical hypothyroidism.

Interpret the TSH level in the context of the low free T4.

The TSH is

2.1 \text{ }\mu\text{U/mL}

, which falls within the normal range.

In primary hypothyroidism (gland failure), a low T4 triggers a massive increase in TSH due to the loss of negative feedback. A normal or low TSH in the presence of a low T4 is inappropriate and indicates central (pituitary or hypothalamic) failure.

Key Concept

Interpretation of thyroid function tests to distinguish primary from central hypothyroidism.

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