A 34-year-old woman comes to the clinic for an annual health maintenance evaluation. She m… — USMLE Step 2 CK

A 34-year-old woman comes to the clinic for an annual health maintenance evaluation. She mentions feeling a bit fatigued lately but otherwise feels well. She has no history of neck radiation, surgery, or significant medical conditions. Her vital signs are within normal limits. Physical examination reveals a normal-sized thyroid gland without palpable nodules.

Laboratory studies are obtained with the following results:

Thyroid-stimulating hormone (TSH): $6.8 \text{ \mu U/mL}$ (Normal: $0.5 - 5.0 \text{ \mu U/mL}$ )
Free thyroxine (T4): $1.4 \text{ ng/dL}$ (Normal: $0.9 - 1.7 \text{ ng/dL}$ )

Which of the following is the most likely diagnosis?

A
Subclinical hyperthyroidism
B
Overt primary hypothyroidism
Subclinical hypothyroidismAnswer
D
Central hypothyroidism
E
Euthyroid sick syndrome

Answer

Subclinical hypothyroidism

The correct diagnosis is subclinical hypothyroidism. This condition is biochemically defined by an elevated thyroid-stimulating hormone (TSH) level occurring alongside a normal serum free thyroxine (T4) level. Patients may be completely asymptomatic or have very mild, non-specific symptoms such as fatigue. In this vignette, the patient's TSH of

6.8 \text{ \mu U/mL}

is elevated, but her free T4 is clearly within normal limits, making this the only accurate diagnosis.

Step-by-Step Solution

Evaluate the patient's Thyroid-stimulating hormone (TSH) level.

The TSH is elevated at

6.8 \text{ \mu U/mL}

TSH is the most sensitive initial screening test for primary thyroid dysfunction.

Evaluate the patient's Free thyroxine (T4) level.

The Free T4 is within the normal range at

1.4 \text{ ng/dL}

Free T4 distinguishes between overt and subclinical thyroid disease when TSH is abnormal.

Synthesize the laboratory findings to determine the diagnosis.

An elevated TSH paired with a normal Free T4 indicates a subclinical hypothyroid state.

The thyroid gland is producing adequate T4, but it requires higher-than-normal TSH stimulation from the pituitary to do so.

Key Concept

Subclinical hypothyroidism is defined biochemically by an elevated TSH level in the presence of a normal free T4 level.

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