A 38-year-old woman comes to the outpatient clinic due to worsening fatigue, weight gain, … — USMLE Step 2 CK

A 38-year-old woman comes to the outpatient clinic due to worsening fatigue, weight gain, and cold intolerance over the last 4 months. She also reports that she has not had a menstrual period for 6 months. Her past medical history is notable for a severe postpartum hemorrhage 4 years ago requiring a blood transfusion. Physical examination reveals dry skin and delayed relaxation of the deep tendon reflexes. Laboratory evaluation shows:

Free T4: $0.4\text{ ng/dL}$ (normal: $0.9-1.7\text{ ng/dL}$ )
TSH: $1.2\text{ }\mu\text{U/mL}$ (normal: $0.5-5.0\text{ }\mu\text{U/mL}$ )

Based on these findings, which of the following is the most likely diagnosis?

Central hypothyroidismAnswer
B
Primary hypothyroidism
C
Subclinical hypothyroidism
D
Euthyroid sick syndrome
E
Resistance to thyroid hormone

Answer

Central hypothyroidism

The correct diagnosis is central hypothyroidism. The patient presents with classic symptoms of hypothyroidism (fatigue, cold intolerance, weight gain, delayed reflexes). Her laboratory results show a low free T4, confirming a hypothyroid state. However, her TSH is within the normal range. In primary thyroid failure, a low free T4 removes negative feedback on the pituitary gland, leading to a significantly elevated TSH. An inappropriately normal or low TSH in the presence of a low free T4 indicates a failure of the pituitary gland or hypothalamus to respond properly, known as central (or secondary/tertiary) hypothyroidism. Her history of a severe postpartum hemorrhage requiring a transfusion strongly suggests Sheehan syndrome (ischemic necrosis of the pituitary gland), which is a classic cause of central hypothyroidism and panhypopituitarism (accounting for her amenorrhea).

Step-by-Step Solution

Interpret the free T4 level.

Free T4 is below the normal range (

0.4\text{ ng/dL}

Establishes that the patient is in a hypothyroid state, which matches her symptoms of fatigue, weight gain, and cold intolerance.

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

TSH is

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

, which is within the normal reference range.

Determines whether the hypothyroidism is primary or central. In a normal physiologic state, a low free T4 should stimulate the pituitary to secrete high amounts of TSH. An inappropriately normal TSH indicates central axis failure.

Correlate laboratory findings with the patient's medical history.

The history of severe postpartum hemorrhage suggests Sheehan syndrome (postpartum pituitary necrosis).

Confirms the diagnosis of a pituitary disorder (central hypothyroidism), which also explains her amenorrhea due to a concurrent loss of gonadotropins.

Key Concept

Central (secondary) hypothyroidism is characterized by a low free T4 level with an inappropriately normal or low TSH level, often caused by pituitary pathology.

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