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Zorluk: Çok zorEndocrinology and Metabolism

A 2828-year-old woman comes to the clinic complaining of palpitations, excessive sweating, and a 5 kg5\text{ kg} (11 lb11\text{ lb}) weight loss over the past 66 weeks despite an increased appetite. She has a history of major depressive disorder, for which she takes fluoxetine. She is a competitive bodybuilder and reports feeling immense pressure for an upcoming competition. Vital signs show a temperature of 37.2∘C37.2^\circ\text{C} (99.0∘F99.0^\circ\text{F}), blood pressure of 145/75 mmHg145/75\ \text{mmHg}, and a pulse of 112/min112\text{/min}. Physical examination reveals a fine resting tremor, brisk deep tendon reflexes, and warm, moist skin. The thyroid gland is nonpalpable, and there is no exophthalmos.

Laboratory results are as follows:

* Thyroid-stimulating hormone (TSH): <0.01 μU/mL<0.01 \ \mu\text{U/mL} (Normal: 0.4−4.0 μU/mL0.4-4.0 \ \mu\text{U/mL})
* Free T4: 0.3 ng/dL0.3 \ \text{ng/dL} (Normal: 0.9−2.4 ng/dL0.9-2.4 \ \text{ng/dL})
* Free T3: 8.5 pg/mL8.5 \ \text{pg/mL} (Normal: 2.0−4.4 pg/mL2.0-4.4 \ \text{pg/mL})
* Radioactive iodine uptake (2424-hour): 1%1\% (Normal: 10%−30%10\%-30\%)

Which of the following additional findings is most likely present in this patient?

  1. Decreased serum thyroglobulin concentrationCevap
  2. B
    Elevated serum reverse triiodothyronine (rT3) concentration
  3. C
    Diffuse granulomatous infiltration of the thyroid follicles
  4. D
    Decreased serum sex hormone-binding globulin (SHBG) concentration
  5. E
    A mass in the sella turcica on magnetic resonance imaging

Cevap

Decreased serum thyroglobulin concentration
This patient presents with clinical thyrotoxicosis (palpitations, weight loss, tremor) confirmed by a suppressed TSH. However, she has a highly specific laboratory pattern: elevated Free T3, decreased Free T4, and a suppressed radioactive iodine uptake (RAIU). This combination is pathognomonic for exogenous liothyronine (synthetic T3) abuse. The exogenous T3 causes clinical hyperthyroidism and strongly suppresses pituitary TSH secretion via negative feedback. The lack of TSH stimulation renders the thyroid gland completely inactive, leading to a near-zero RAIU and a cessation of endogenous T4 and thyroglobulin synthesis. Therefore, the serum thyroglobulin concentration will be markedly decreased. Her background as a competitive bodybuilder suggests she is using liothyronine surreptitiously to rapidly accelerate fat loss before a competition.

Adım Adım Çözüm

1
Analyze the patient's clinical presentation.
The patient has signs and symptoms of thyrotoxicosis, including weight loss, palpitations, tremor, hyperreflexia, and tachycardia.
Establishing the clinical state (hyperthyroid vs hypothyroid) is the first step in interpreting complex thyroid function tests.
2
Interpret the thyroid function tests.
TSH is suppressed, confirming primary thyrotoxicosis. However, there is a discordant T4/T3 split: Free T3 is markedly elevated, while Free T4 is abnormally low.
Almost all endogenous causes of thyrotoxicosis (Graves' disease, toxic nodule, thyroiditis) elevate both T4 and T3. A low T4 with a high T3 strongly narrows the differential.
3
Evaluate the radioactive iodine uptake (RAIU).
The RAIU is 1% (severely decreased).
A low RAIU indicates that the thyroid gland is entirely inactive and is not synthesizing new hormone, ruling out Graves' disease or toxic adenomas.
4
Synthesize findings to determine the underlying etiology.
The combination of thyrotoxicosis, low Free T4, high Free T3, and low RAIU is pathognomonic for exogenous liothyronine (synthetic T3) abuse (factitious thyrotoxicosis).
Exogenous T3 suppresses TSH. The lack of TSH stimulation prevents the thyroid gland from synthesizing endogenous T4, causing Free T4 to fall.
5
Determine the effect of this condition on thyroglobulin.
Serum thyroglobulin concentration will be decreased.
Thyroglobulin is synthesized exclusively by active thyroid follicles. Because the gland is completely suppressed by exogenous T3, thyroglobulin production ceases.

Anahtar Kavram

Exogenous liothyronine (T3) abuse presents with thyrotoxicosis, suppressed TSH, elevated Free T3, decreased Free T4, low radioactive iodine uptake, and decreased serum thyroglobulin.
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