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Difficulty: MediumEndocrinology and Metabolism

An otherwise healthy 29-year-old fitness instructor presents for medical evaluation due to a one-month history of a racing heart, heat intolerance, and shakiness. She notes that she has been taking multiple non-prescription nutritional supplements to accelerate fat loss for an upcoming bodybuilding competition. Vital signs demonstrate a blood pressure of 135/75 mm Hg and a heart rate of 112/min. Physical examination reveals a fine tremor in her outstretched hands, hyperreflexia, and warm, moist skin. There is no exophthalmos, and her thyroid gland is nonpalpable.

Laboratory studies show the following:

Laboratory testResultReference range
Thyroid-stimulating hormone (TSH)<0.01 \muU/mL<0.01 \text{ \mu U/mL}0.5−5.0 \muU/mL0.5-5.0 \text{ \mu U/mL}
Free thyroxine (T4)0.5 ng/dL0.5 \text{ ng/dL}0.9−1.7 ng/dL0.9-1.7 \text{ ng/dL}
Total triiodothyronine (T3)280 ng/dL280 \text{ ng/dL}80−200 ng/dL80-200 \text{ ng/dL}

Which of the following is the most likely diagnosis?

  1. A
    Central hypothyroidism
  2. B
    Graves disease
  3. Surreptitious liothyronine ingestionAnswer
  4. D
    Painless thyroiditis
  5. E
    Surreptitious levothyroxine ingestion

Answer

Surreptitious liothyronine ingestion
The correct answer is surreptitious liothyronine ingestion. This patient presents with overt clinical signs of thyrotoxicosis (tachycardia, tremor, heat intolerance) and a suppressed TSH level. However, her peripheral hormone profile shows an unusually discordant pattern: markedly elevated T3 but a low free T4. This specific pattern is pathognomonic for exogenous liothyronine (synthetic T3) use, which is sometimes taken surreptitiously as a weight loss or bodybuilding aid. The exogenous T3 floods the system, causing clinical thyrotoxicosis and providing strong negative feedback to the pituitary gland, thereby suppressing TSH secretion. Because TSH is suppressed, the thyroid gland is not stimulated to produce endogenous thyroid hormones, resulting in a low free T4 level and a physically nonpalpable thyroid gland.

Step-by-Step Solution

1
Evaluate the clinical presentation and TSH level.
The patient has distinct signs of thyrotoxicosis (tachycardia, tremor, hyperreflexia) combined with a completely suppressed TSH level (<0.01 \muU/mL<0.01 \text{ \mu U/mL}).
This establishes that the patient is thyrotoxic and rule out secondary (central) causes of hyperthyroidism.
2
Analyze the peripheral thyroid hormone pattern (Free T4 and Total T3).
The free T4 is abnormally low (0.5 ng/dL0.5 \text{ ng/dL}), while the Total T3 is significantly elevated (280 ng/dL280 \text{ ng/dL}).
This discordant pattern (high T3 but low T4) is crucial for differentiating the specific etiology of the thyrotoxicosis.
3
Synthesize findings to determine the exact diagnosis.
The combination of clinical thyrotoxicosis, suppressed TSH, high T3, low T4, and a nonpalpable thyroid confirms exogenous T3 (liothyronine) intake, likely from bodybuilding supplements.
Exogenous T3 causes systemic thyrotoxicosis and suppresses pituitary TSH. The lack of TSH halts endogenous thyroid gland function, resulting in low T4 production.

Key Concept

Thyrotoxicosis combined with a low free T4 and high T3 level is the hallmark pattern of exogenous liothyronine (T3) ingestion.
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Topics

Endocrinology and Metabolism

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