This fascinating case report by Huang and colleagues, from Children's Hospital in Boston, Massachusetts, which was initially presented in abstract form at this conference, but was recently published in the New England Journal of Medicine,describes a case of primary hypothyroidism resulting from high expression of type-3 iodothyronine deiodinase activity in hepatic hemangioma tissue. Type-3 iodothyronine deiodinase, a selenoenzyme that catalyzes the inactivation of thyroxine (T4) and triiodothyronine (T3), is normally present in brain and placenta.
An infant male presenting with a large hepatic hemangioma is diagnosed with hypothyroidism at 6 weeks of age. The thyroid-stimulating hormone (TSH) level at that time was 156 mcU/mL (the normal range value is less than 6.2 mcU/mL). Serum free T4 was also low. At the request of the parents, no newborn screening for hypothyroidism had been performed. Levothyroxine was administered at a dose of 37.5 mcg/day.
At 3 months of age, the infant presented with bradycardia, hypothermia, and abdominal distention. The serum level of TSH was 177 mcU/mL, T4 was 2.5 mg/dL, T3 was less than 15 ng/dL, and reverse T3 (rT3) was increased to 413 ng/dL. The serum TSH remained high despite administering higher doses of levothyroxine. Ultimately, an intravenous infusion of levothyroxine and liothyronine was required to decrease the serum TSH to approximately the normal range. As much as approximately 100 mcg liothyronine and 50 mcg levothyroxine were required to decrease TSH. These doses are approximately 8-9 times those used to treat congenital hypothyroidism. Ultimately, although the T3 level normalized, rT3 remained high, and T4 failed to increase significantly.
The extremely unusual aspects of this case, including the severe hypothyroidism, apparent resistance to exogenous levothyroxine, and the inability to increase T4 suggested that there might be an increased rate of inactivation of T4 and T3. Huang and colleagues subsequently demonstrated that the hepatic hemangioma expressed a high level of type-3 iodothyronine deiodinase activity that was approximately 7.5-fold higher than what is normally present in term placenta (the human tissue with the highest levels of type-3 iodothyronine deiodinase activity). Northern blot analysis and in situ hybridization confirmed that type-3 iodothyronine deiodinase messenger RNA was expressed by hemangioma cells. Five additional patients with hemangiomas were studied, and the presence of type-3 iodothyronine deiodinase activity was confirmed in 3 cases. A retrospective analysis of 92 patients with hemangiomas identified 9 children who had elevated TSH levels and 3 patients who had markedly elevated levels.
This report describes, for the first time, the case of hepatic hemangiomas generating enough type-3 iodothyronine deiodinase activity to produce clinically relevant decreases in serum T4 and T3 and, therefore, high levels of TSH. This is in conjunction with hypothyroidism that is resistant to customary doses of levothyroxine. Thyroid-function screening of children with hemangiomas similar to those reported in this study is recommended.
Lubiniecki, from Mayo Clinic in Rochester, Minnesota, presented the case of a 66-year-old woman who had a several-year history of progressive dyspnea. Physical examination revealed a systolic ejection murmur, and echocardiogram demonstrated obstruction of right ventricular outflow by a mass attached to the interior ventricular septum. Surgical exploration uncovered a 3-cm mass attached to the ventricular septum and the papillary muscle. There was no evidence of invasion. Histologic examination of the tissue revealed thyroid follicles that were apparently normal by histologic criteria. The patient had a slightly increased TSH (5.8 mcU/mL) and a slightly decreased T4 (0.6 ng/dL).
This represents an extremely unusual case of intracardiac thyroid tissue. The author reviewed the literature from January 1966 to the year 2000 and only uncovered 20 additional reported cases. Most of these patients presented with fatigue, dyspnea, palpitations, or syncope. Although the present case is unusual, the possible existence of intracardiac thyroid tissue should be considered in the differential diagnosis of the aforementioned symptoms, most notably dyspnea and a cardiac mass.
Frequently, it is difficult to discern malignant from benign ectopic thyroid tissue. Apparently this was benign tissue, and in some cases the ectopic tissue can represent the sole thyroid tissue in that individual. Such patients should have thyroid function evaluated following surgery. Ectopic tissue can be found in other locations outside of the neck such as the ovaries, and the unusual syndrome struma ovarii is well recognized.