A 46-year-old woman comes to the office for follow-up. Six months ago, the patient underwent total thyroidectomy followed by radioactive iodine ablation for invasive follicular thyroid cancer with distant metastasis. After surgery, she was prescribed levothyroxine to prevent thyroid cancer regrowth. Over the past 4 months, the patient has had a 4.5-kg (10 lb) unintentional weight loss despite having a normal appetite. She also feels unusually hot and sweaty. Temperature is 37 C (98.6 F), blood pressure is 150/70 mm Hg, pulse is 110/min, and respirations are 17/min. Examination reveals a fine hand tremor and sweaty palms. Serum thyroid function testing would most likely reveal which of the following?
Exogenous thyrotoxicosis | |
Causes |
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Laboratory |
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*Use of T3 supplement (liothyronine) shows elevated T3 only. T3 = triiodothyronine. | |
This patient has thyrotoxicosis (eg, weight loss, tremor, tachycardia) due to suppressive-dose levothyroxine therapy. Because TSH can promote growth of residual malignant cells following thyroidectomy, patients with thyroid cancer at high risk of recurrence (eg, known metastasis) are often prescribed levothyroxine doses that are higher than what is needed for simple replacement. These higher doses adequately suppress pituitary secretion of TSH, however, they can also cause symptomatic thyrotoxicosis.
When exogenous thyrotoxicosis is due to levothyroxine or mixed T3/T4 supplements (eg, porcine thyroid extract), total and free thyroxine (T4) levels are elevated; triiodothyronine (T3) is produced primarily by deiodination of T4 in peripheral tissues, so T3 levels also are elevated, even if the patient is taking levothyroxine only (Choice F). In contrast, thyrotoxicosis due to the intake of liothyronine (exogenous T3) is associated with high serum T3 and low T4 (Choice B).
(Choice C) Subclinical hyperthyroidism is characterized by a mildly suppressed TSH with normal thyroid hormone levels. Patients with subclinical hyperthyroidism are typically asymptomatic. This patient takes suppressive-dose levothyroxine to manage metastatic thyroid cancer and has overt hyperthyroid symptoms; thyroid hormone levels are likely to be elevated.
(Choice D) Patients with severe, acute illness may develop thyroid function abnormalities resembling a mild central hypothyroid state, with low TSH, normal T4, and low T3 (ie, euthyroid sick syndrome). Patients receiving suppressive-dose levothyroxine typically have normal or elevated T3 due to peripheral deiodination.
(Choice E) Central/secondary hyperthyroidism (eg, TSH-secreting pituitary adenoma) is characterized by elevated TSH and thyroid hormone levels. Patients receiving suppressive-dose levothyroxine have low TSH due to feedback inhibition of TSH release.
Educational objective:
Exogenous thyrotoxicosis can be caused by suppressive levothyroxine therapy for thyroid cancer. When exogenous thyrotoxicosis is due to levothyroxine or mixed T3/T4 supplements, thyroxine (T4) levels are elevated; triiodothyronine (T3) is produced primarily by the peripheral deiodination of T4, so T3 levels also are elevated even if the patient is taking T4 only.