A 32-year-old female nurse presents to the clinic reporting a 6-week history of persistent… — USMLE Step 2 CK

A $32$ -year-old female nurse presents to the clinic reporting a $6$ -week history of persistent jitteriness, heat intolerance, and difficulty sleeping. She also reports a $4\text{-kg}$ ( $8.8\text{-lb}$ ) weight loss during this period despite a normal appetite. The patient has no significant past medical history. She mentions feeling significant stress recently due to her upcoming wedding. Her temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $138/78\text{ mm Hg}$ , and pulse is $115/\text{min}$ . On physical examination, she appears anxious and diaphoretic. A fine resting tremor is observed in her outstretched hands. The thyroid gland is nonpalpable and non-tender. There is no cervical lymphadenopathy, proptosis, or pretibial myxedema. Laboratory studies are obtained and reveal the following:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$0.01\text{ \mu U/mL}$	$0.4-4.2\text{ \mu U/mL}$
Free thyroxine (T4)	$0.4\text{ ng/dL}$	$0.9-1.7\text{ ng/dL}$
Total triiodothyronine (T3)	$310\text{ ng/dL}$	$80-200\text{ ng/dL}$

Which of the following is the most likely diagnosis?

A
Central hypothyroidism
B
Graves disease
C
Subacute granulomatous thyroiditis
D
Surreptitious levothyroxine ingestion
Surreptitious liothyronine ingestionAnswer

Answer

Surreptitious liothyronine ingestion

The patient's clinical presentation and suppressed TSH indicate thyrotoxicosis. However, her free T4 is frankly low while her T3 is highly elevated. This discordant laboratory pattern is the hallmark of exogenous T3 (liothyronine) ingestion. The exogenous T3 exerts strong negative feedback on the pituitary, completely suppressing TSH. Without TSH stimulation, the endogenous thyroid gland halts the synthesis and release of T4. Therefore, the circulating T3 is high (from the ingested pills), while TSH and T4 are low. The patient's profession as a nurse provides her with medical access, and her desire to lose weight for a wedding provides a motive for factitious thyrotoxicosis.

Step-by-Step Solution

Analyze the clinical presentation and physical examination.

The patient has classic signs of thyrotoxicosis (weight loss, tachycardia, heat intolerance, tremor, diaphoresis) with a nonpalpable thyroid gland.

This establishes that the patient is in a hypermetabolic state and rules out large goiters or classically painful thyroiditis.

Interpret the laboratory data in the context of the hypothalamic-pituitary-thyroid axis.

The suppressed TSH (

0.01\text{ \mu U/mL}

) confirms thyrotoxicosis. The elevated T3 (

310\text{ ng/dL}

) is driving the symptoms. However, the free T4 (

0.4\text{ ng/dL}

) is abnormally low.

Identifying the discordant pattern between T3 and T4 is critical for isolating the etiology of the thyrotoxicosis.

Determine the pathophysiological mechanism responsible for the lab pattern.

A suppressed TSH with a low free T4 and high T3 indicates an exogenous source of pure T3 (liothyronine).

Exogenous T3 suppresses pituitary TSH secretion via negative feedback. The lack of TSH stimulation halts the endogenous thyroid gland's production of T4, leading to a low serum T4 level.

Key Concept

Factitious thyrotoxicosis from exogenous T3 (liothyronine) presents with clinical hyperthyroidism, suppressed TSH, elevated T3, and a uniquely low free T4 due to suppression of endogenous thyroid hormone synthesis.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic