Uncommon Causes of Thyrotoxicosis

jnm041202 265..278C O N T I N U I N G E D U C A T I O N
Uncommon Causes of Thyrotoxicosis*
Erik S. Mittra1, Ryan D. Niederkohr1, Cesar Rodriguez1, Tarek El-Maghraby2,3, and I. Ross McDougall1
1Division of Nuclear Medicine and Molecular Imaging Program at Stanford, Department of Radiology, Stanford University Hospital and Clinics, Stanford, California; 2Nuclear Medicine, Cairo University, Cairo, Egypt; and 3Nuclear Medicine, Saad Specialist Hospital, Al Khobar, Saudi Arabia
Apart from the common causes of thyrotoxicosis, such as Graves’ disease and functioning nodular goiters, there are more than 20 less common causes of elevated free thyroid hor- mones that produce the symptoms and signs of thyrotoxicosis. This review describes these rarer conditions and includes 14 il- lustrative patients. Thyrotropin and free thyroxine should be measured and, when the latter is normal, the free triiodothyronine level should be obtained. Measurement of the uptake of 123I is recommended for most patients.
Key Words: thyrotoxicosis; Graves’ disease; thyroiditis; thyroid hormones
J Nucl Med 2008; 49:265–278 DOI: 10.2967/jnumed.107.041202
Thyrotoxicosis is the syndrome caused by an excess of free thyroid hormones. Any or all systems of the body can be affected. The symptoms and signs depend on the degree of elevation of the hormones, the length of time that they have been elevated, the rate at which the hormone levels rose, and individual variations of patients. For example, a patient with ischemic heart disease is more likely to exhibit cardiac manifestations. The terms ‘‘hyperthyroidism’’ and ‘‘thyro- toxicosis’’ are often used interchangeably; however, hyper- thyroidism means that the thyroid gland is functioning more than normal. Therefore, a hyperthyroid patient is thyro- toxic, but a thyrotoxic patient need not have an overactive thyroid and is therefore not actually hyperthyroid. Iagaru and McDougall discussed the treatment of thyrotoxicosis, focusing on common diseases, such as Graves’ disease and toxic nodular disorders (1). In this article, we describe rare causes of thyrotoxicosis. Nuclear medicine physicians will not encounter these often; however, knowledge of the disorders, including how to diagnose and manage them, is important.
Several of the conditions are self-limiting and do not need prolonged treatment.
When a patient is thought to be thyrotoxic, a convenient algorithm is to measure free thyroxine (free T4) and thyrotropin (TSH). When the former is higher than normal but the latter is suppressed, thyrotoxicosis is diagnosed. When the former is normal but TSH is low, it is valuable to measure free triiodothyronine (free T3); when the latter is abnormally high, the diagnosis is T3 toxicosis (2–4). When both free hormones are normal but TSH is low, the term ‘‘subclinical thyrotoxicosis’’ can be applied (5). Once it has been determined that thyrotoxicosis is present, measure- ment of 123I uptake can differentiate among several disor- ders (Table 1). Some have high uptake and, paradoxically, some have low uptake. This distinction is important be- cause most disorders in the latter group are self-limiting and do not require treatment with 131I or antithyroid medi- cations. In contrast, those with high uptake usually do re- quire therapy. It should be recognized that the site of high uptake might not be in the expected site of the thyroid.
Conditions associated with elevated uptake of radioiodine are addressed first (6). Several disorders are illustrated by case reports. The description of each group of thyrotoxic conditions is followed by a discussion relevant to those specific disorders. At the end of the article, there is no repetition of the discus- sions, but there is a ‘‘Conclusion’’ section. Routine cases of Graves’ disease and toxic nodular goiters are not discussed.
UNCOMMON CAUSES OF THYROTOXICOSIS WITH INCREASED UPTAKE OF RADIOIODINE
Thyrotoxicosis in Newborns
Examples. Patient 1 was a newborn baby who was irritable and restless. He had sinus tachycardia. Although the mother had been a patient at Stanford University Medical Center, the obstetric care and delivery were pro- vided at a different medical facility. Four days after delivery, the physicians noted that the mother had a thyroidectomy scar and determined that she had been treated for Graves’ hyperthyroidism. She had a distant history of weight loss, shaking, palpitations, sweating, and irritation of her eyes. Free T4 was high, TSH was suppressed, and she had been treated with propylthiouracil. She developed severe thyroid orbitopathy, which was treated with high doses of prednisone
Received Feb. 26, 2007; revision accepted Sep. 10, 2007. For correspondence or reprints contact: I. Ross McDougall, Stanford
University Hospital and Clinics, 300 Pasteur Dr., Room H-0101, Stanford, CA 94305-5281.
E-mail: [email protected] *NOTE: FOR CE CREDIT, YOU CAN ACCESS THIS ACTIVITY THROUGH
THE SNM WEB SITE (http://www.snm.org/ce_online) THROUGH FEBRUARY 2009.
No potential conflict of interest relevant to this article was reported. COPYRIGHT ª 2008 by the Society of Nuclear Medicine, Inc.
UNCOMMON CAUSES OF THYROTOXICOSIS • Mittra et al. 265
and external beam radiation. She also had pretibial dermo- pathy. The patient elected to have a thyroidectomy so that she could conceive as early as possible because she was 35 y old. The baby was then diagnosed with neonatal Graves’ disease, which was confirmed by thyroid function tests. Propylthiouracil and propranolol were administered for 4 wk. The baby then required no therapy for thyrotoxicosis. In retrospect, in utero the baby had been noted to have per- sistent tachycardia. This baby has neonatal Graves’ disease.
Patient 2 had failure to thrive from birth. It was consid- ered that the mother was not feeding him, but thyrotoxico- sis was diagnosed when he was 8 wk of age. Antithyroid antibodies, including antithyroglobulin, antithyroperoxi- dase, and thyroid-stimulating immunoglobulin (TSI), were negative. His mother had an enlarged thyroid but normal thyroid function and no history of Graves’ hyperthyroidism.
The baby was treated with antithyroid medications but remained thyrotoxic for 3 y and had a thyroidectomy at that age. He continued to be thyrotoxic, with elevated free T4
and suppressed TSH, and was treated with methimazole. As a result of persistent fetal and neonatal thyrotoxicosis, the patient had premature closure of skull sutures and deafness. At 12 y of age, a decision was made to treat the patient with radioiodine. Methimazole was stopped for 5 d, and imaging of the neck and trunk with 123I demonstrated uptake only in the thyroid bed. He was treated with 460 MBq (12.5 mCi) of 131I and is now taking replacement L-thyroxine. This patient has constitutively activated TSH receptors.
Discussion. Thyrotoxicosis in neonates is very uncommon (7). When a baby is exposed to high levels of free thyroid hormones in utero, it is at increased risk of being born pre- maturely or small, and fetal mortality is increased. Premature closure of cranial sutures and reduced intelligence are noted, and cardiac failure can occur. The most likely cause of hyper- thyroidism in a newborn is neonatal Graves’ disease attribut- able to passive transplacental transfer of thyroid-stimulating antibodies from mother to baby, as was the case in patient 1. In this siuation, the maternal level of TSI, an IgG antibody, is high and remains elevated during the pregnancy (8–10). In contrast, in most pregnant patients with Graves’ disease, the levels of TSI decrease as the pregnancy progresses. Maternal antibodies, both good and bad, are passively transferred to the fetus. The mother can be euthyroid and, if so, usually has been treated for Graves’ hyperthyroidism (11). Guidelines indicate that mea- surement of TSH receptor antibodies should be obtained in a patient who has had surgery or 131I for Graves’ hyperthyroid- ism or who is taking antithyroid medications during pregnancy (12). Management of the baby in utero is beyond the scope of this article, but when the diagnosis is suggested, careful monitoring of the fetal size and heart rate and the size of the fetal thyroid by ultrasound is important (13).
Although the second patient was hyperthyroid from birth, he does not have neonatal Graves’ disease. His mother did not have Graves’ disease, there was no evidence of thyroid autoimmunity or abnormal thyroid function, and his disease persisted despite antithyroid medications and thyroidectomy. The cause is a mutation, usually substitution of one base in the DNA responsible for the production of the TSH receptor or the related G protein complex. This muta- tion results in activation of the TSH receptor without the presence of TSH or TSH-like stimulators, such as thyroid- stimulating antibodies. The patient now takes L-thyroxine after 131I therapy. Thyrotoxicosis in the case of an activated TSH receptor or G protein is permanent until all thyroid tissue has been removed (9,14,15).
A third cause of thyrotoxicosis in a newborn is the transfer of thyroid-stimulating antibodies in the mother’s milk. We have not encountered this situation in practice (16).
Excess TSH
Examples. Patient 3 was 41 y old when she was found to be thyrotoxic during pregnancy. Free T4 and free T3 were
TABLE 1 Unusual Causes of Thyrotoxicosis
Increased uptake of
TSI in milk Silent thyroiditis
Excess TSH Recurrent silent thyroiditis
Pituitary tumor Familial silent thyroiditis Resistance to thyroid
hormone
Choriocarcinoma Internal radiation
External radiation
Struma ovarii Carcinomatous
Thyrotoxicosis factitia
veterinarius Thyrotoxicosis
Amiodarone types I and II
Iodine supplementation
Miscellaneous Lithium
(transfer of TSI)
266 THE JOURNAL OF NUCLEAR MEDICINE • Vol. 49 • No. 2 • February 2008
high, and TSH was normal. She had a diffusely enlarged thyroid. She was treated with propylthiouracil. Fourteen months after delivery, she was still hyperthyroid, and because TSH was measurable, there was a concern that the condition was secondary to a pituitary tumor. An MRI of the pituitary showed a macroadenoma, and she under- went transsphenoidal surgery that turned out to be incom- plete. She continued to be thyrotoxic clinically and had elevated free hormones, but there was evidence of persis- tent TSH secretion. This patient does not have Graves’ disease, but the cause of the hyperthyroidism is persistent TSH secretion from a pituitary adenoma.
Patient 4 was 55 y old when he was treated for thyroid cancer with thyroidectomy and 131I. Thereafter, the goal was to administer a dose of L-thyroxine that would keep his TSH at the low end of the normal range. Over a decade, his thyroid tests have consistently shown high free T4 and normal or high TSH (Table 2 shows representative values over 10 y). The patient is clinically mildly thyrotoxic, and he developed atrial fibrillation that is permanent. He is nervous and has difficulty sleeping. He has elevated free T4
at times when his TSH is well within normal or even ele- vated. At these times, he has worsening of the thyrotoxic symptoms. Follow-up whole-body scintiscans and serum thyroglobulin measurements indicate that there has been no recurrence of the cancer. This patient has pituitary resis- tance to thyroid hormones.
Discussion. These cases highlight examples of high TSH (17–19). TSH-secreting pituitary tumors are rare, occurring at a rate of approximately 1 in 1,000,000 in the general population and representing 1% or less of all pituitary ade- nomas (19–21). Most secrete TSH alone, although mixed hormonal secretion (growth hormone, prolactin and, rarely, gonadotropins) occurs in approximately 30% (22,23). Men and women are affected nearly equally. TSH-secreting ade- nomas have been associated with both multiple endocrine
neoplasia type I and McCune–Albright syndrome (19,20). Most patients have symptoms of thyrotoxicosis for years before the diagnosis of a TSH-secreting pituitary tumor (21). The thyroid gland is palpably enlarged and often multi- nodular because of sustained TSH stimulation. Visual field defects (classically bitemporal hemianopia) are present in approximately 40%–50% of cases because of compression of the optic chiasm by the pituitary tumor. The key to the diagnosis of hyperthyroidism attributable to a pituitary ade- noma is elevated thyroid hormone levels and detectable TSH levels. The biologic activity of the secreted TSH varies considerably; thus, serum immunoreactive TSH concentra- tions may range from normal (although inappropriately high given the presence of high free hormone levels) to markedly elevated (.500 mU/L). a-Subunit hypersecretion is seen in 66% of patients, and the molar ratio of the a-subunit to TSH is elevated in 80% of patients (24,25). In conjunction with typical laboratory results, MRI of the pituitary showing a mass lesion is usually diagnostic. Rarely, inferior petrosal sinus sampling for TSH is indicated when the results of MRI of the pituitary are normal. The most effective therapy is transsphe- noidal resection of the pituitary tumor, which results in cure, improvement, or no change each in about one third of patients (25,26). External radiation and long-acting somatostatin ana- logs have been used (27–29). Treatment focused on the thyroid results in continued growth of the pituitary tumor. Neverthe- less, there are reports of ablation of the thyroid rather than therapy of the pituitary adenoma, but careful long-term mon- itoring of the pituitary is important (30).
A particularly rare case was reported by Cooper and Wenig (31). The evidence suggested a TSH-secreting tumor, but studies of the pituitary were normal. The TSH was secreted by an ectopic nasopharyngeal pituitary tumor that was identified when the patient developed nasal obstruction.
The major differential diagnosis is resistance to thyroid hormones, which may be generalized or, in rare cases, se- lective to the pituitary gland (32,33). If the results of MRI of the pituitary are equivocal, then the differential diagnosis between a TSH-secreting tumor and resistance to thyroid hormones is made by administering thyrotropin-releasing hormone (TRH) (note that TRH is not available in the United States). Patients with a pituitary adenoma that autonomously produces TSH will have a blunted or absent TSH response to TRH stimulation, most likely because of the lack of TRH receptors on the adenoma itself (32). In addition, serum sex hormone–binding globulin concentra- tions are typically high in patients with TSH-secreting pituitary adenomas but normal in the syndrome of resis- tance to thyroid hormones.
Patient 4 appeared to have resistance to thyroid hor- mones at the pituitary level (34). His free hormone levels are normal or high, he has symptoms and signs of thyro- toxicosis, and his TSH levels are also normal or high. This condition can be difficult to manage, especially in a case like this one, in which the optimal TSH level for treating the thyroid cancer would be at the low end of the normal
TABLE 2 Free T4 and TSH Values in Patient Taking Exogenous
Thyroxine and Exhibiting Symptoms and Signs of Thyrotoxicosis
Free T4 (ng/dL)* TSH (IU/L)y
1.0 9.1
2.0 2.3
2.2 3.8
2.7 3.4
1.8z 3.7
*Normal range 5 0.7–2.0. yNormal range 5 0.4–4.0. zUpper limit of this assay 5 1.6 ng/dL.
UNCOMMON CAUSES OF THYROTOXICOSIS • Mittra et al. 267
range. Another cause of thyrotoxicosis and elevated TSH has been reported by Pishdad et al (35). Their patient had been treated with methimazole for 14 y. Three months after discontinuation of the medication, the patient became thyrotoxic, but TSH was high. The condition spontaneously returned to normal. Their explanation was that the pituitary was hyperplastic after years of excess antithyroid therapy (the TSH level was 140 mU/L) and there continued to be an outpouring of TSH that caused hyperthyroidism.
In the presence of elevation of thyroid hormones, typical features of Graves’ disease, and inappropriately high TSH but no evidence of pituitary disease, laboratory misinfor- mation should be considered. Such misinformation could include heterophile antibodies interfering with the TSH assay (17).
Excess TSH-Like Material and Gestational Trophoblastic Diseases
Human chorionic gonadotropin (hCG) is a glycoprotein hormone that shares a common a-subunit with TSH, follicle-stimulating hormone, and luteininzing hormone but that has a specific b-subunit. hCG is synthesized primarily by syncytiotrophoblastic tissue, maintaining the corpus luteum and progesterone production in pregnancy (36,37). Receptors for the structurally similar glycoprotein hormones share significant homology in their extracellular binding domains, and hCG has confirmed thyroid-stimulating activity when present at high concentrations in serum be- cause of a direct interaction of hCG with the TSH receptor (37–42).
Hydatidiform moles secrete large amounts of hCG, and the levels are proportional to the mass of the tumor. The values are severalfold higher than normal and can stimulate thyroid function (43). hCG secreted by a mole can also have greater thyrotropic potency than hCG secreted in normal pregnancy. Increased thyroid function in patients with hydatidiform moles can occur in 25%–64% of cases, but only 5% of cases have clinically significant thyrotox- icosis (44–46). Hydatidiform moles are most common in Asian and Latin American patients. In the United States, they occur in 0.5–2.5 per 1,000 pregnancies. The clinical presentation is usually vaginal bleeding, and the uterus is large for the date of the pregnancy. The diagnosis is con- firmed by ultrasound, showing a pathognomonic ‘‘snowstorm’’ pattern and the absence of a fetus. Definitive treatment requires uterine evacuation of molar material by suction curettage or hysterotomy; the rapid decline in serum hCG levels is accompanied by a parallel decline in serum thyroid hormone levels.
Choriocarcinoma is a malignant tumor that is patholog- ically characterized by sheets of syncytiotrophoblastic and cytotrophoblastic cells, necrosis, and the absence of hy- dropic villi. Choriocarcinoma can invade blood vessels and progress to hemorrhagic metastases in distant organs such as the lungs and vagina. Its prevalence is 1 in 20,000 to 1 in 40,000 pregnancies in the United States and Europe (47).
Approximately one half of choriocarcinomas occur in women who have had previous hydatidiform moles; how- ever, only 3%–5% of women with moles develop chorio- carcinomas (48). Rare cases of hyperthyroidism have also been reported in men with testicular choriocarcinoma (49). The usual treatment is chemotherapy and, as with a hyda- tidiform mole, cure of the choriocarcinoma results in a decline in hCG levels, and cure resolves the thyrotoxicosis. It can be necessary to treat the symptoms and signs of thyrotoxicosis before considering surgery (50).
New-onset thyrotoxicosis in pregnancy is usually attrib- utable to Graves’ disease, but if the clinical features are atypical and evidence of autoimmune thyroid disease is lacking, measurement of hCG and pelvic ultrasound are rec- ommended. For completeness, it should be recognized that there can be a transient drop in TSH in normal pregnancy or in a pregnancy complicated by hyperemesis gravidarum because of high levels of hCG (51–53). When the free hormone levels are normal, the TSH level is low, and the patient has no symptoms or signs of thyrotoxicosis, it is wise to wait 3–4 wk and repeat the tests. If the TSH level im- proves, then no antithyroid therapy is advised.
High Uptake in Ectopic Sites
Struma ovarii is a teratoma of the ovary that is composed primarily of thyroid epithelium which, by definition, com- prises more than 50% of its structure (54). Struma ovarii was first described in 1889, and fewer than 500 cases have been reported in the literature (55). Between 15% and 20% of ovarian tumors are of germ cell origin, and approxi- mately 10% of these contain thyroid cells. A smaller percentage contain sufficient thyroid cells to be classified as struma ovarii (56). The lesion represents 0.3%–1% of all ovarian neoplasms and 2%–4% of all ovarian teratomas (57). It is most common in the fifth or sixth decade and is seen more often in countries in which goiter is endemic (58). Most struma ovarii lesions are benign, and it has been estimated that fewer than 3% are malignant (56).
Kempers et al. described 3 clinical situations for struma ovarii, asymptomatic, ascitic, and thyrotoxic (59). Ascites is evident in one third of the cases. Approximately 5%– 20% of strumae produce significant amounts of thyroid hormones (60,61). The diagnosis should be considered in a patient with thyrotoxicosis, low radioiodine uptake over the thyroid, and a pelvic mass. Suspicion should be also raised in a patient who is thought to have silent thyroiditis and who fails to experience a hypothyroid phase (61). Most benign strumae accumulate iodine and occasionally (5%– 20%) may even produce significant amounts of thyroid hormones and cause thyrotoxicosis. The common scenario is…