Contrast induced hyperthyroidism due to iodine excess

Abstract

Iodine induced hyperthyroidism is a thyrotoxic condition caused by exposure to excessive iodine. Historically this type of hyperthyroidism has been described in areas of iodine deficiency. With advances in medicine, iodine induced hyperthyroidism has been observed following the use of drugs containing iodine—for example, amiodarone, and contrast agents used in radiological imaging. In elderly patients it is frequently difficult to diagnose and control contrast related hyperthyroidism, as most of these patients do not always present with typical signs and symptoms of hyperthyroidism. Treatment can be very challenging as drugs commonly used to treat hyperthyroidism have little effect on already formed thyroid hormone due to iodine excess.

Background

Iodine induced hyperthyroidism is a well documented phenomenon. Most of the time it is associated with underlying silent thyroid disease following one or two contrast based radiological imagining. Diagnosis can be easily missed as elderly patients do not always present with typical features of hyperthyroidism. The patient described in this case report had no previous thyroid problem and had undergone various iodine containing contrast (ICC) based radiological imaging procedures before presenting with somewhat atypical features of hyperthyroidism, thus highlighting the need to consider thyroid function testing of older patients when undergoing multiple ICC procedures. Furthermore, after failing to respond to anti-thyroid drugs, our patient eventually responded to lithium which could have a more inhibitory effect on secretion of T4 (thyroxine) and T3 (tri-iodothyronine) than propylthiouracil and carbimazole.

Case presentation

A 76-year-man, diagnosed with stage IV adenocarcinoma of the colon, presented to us with a history of falls, lethargy, and weight loss. He underwent right hemicolectomy 2 months earlier and had received two cycles of neoadjuvant FOLFOX (oxaliplatin, flurouracil, leucovarin) chemotherapy before liver resection. He had undergone eight contrast based radiological imaging procedures over a period of 10 months before this presentation, including computed tomography (CT) of the brain, chest, abdomen and pelvis, the last being only 2 weeks earlier. He had a history of hypertension, hyperlipidaemia, ischaemic heart disease, inducible ventricular tachycardia, ventricular fibrillation associated with previous cardiac arrest, and an implantable cardioverter defibrillator (ICD). No prior thyroid disease was noted. He was on sotalol for 12 years and was changed to amiodarone after having multiple episodes of sustained ventricular tachycardia. He remained on amiodarone for almost one-and-half years before it was stopped because of amiodarone related lung disease.

The patient was admitted for further investigations following a sudden deterioration in performance status, weight loss, and falls. Thyroid function test during this admission was consistent with hyperthyroidism (thyroid stimulating hormone (TSH) <0.01 mIU/l, FT4 >75 pmol/l, FT3 >30 pmol/l). On examination the thyroid was not palpable and there were no overt signs of hyperthyroidism aside from weight loss and paroxysmal runs of atrial fibrillation.

Investigations

Further investigations included anti-thyroid antibodies, which were negative, and a thyroid nuclear scan which showed no uptake of tracer within the thyroid gland, supporting iodine overload.

Differential diagnosis

At this stage an endocrinologist review occurred and the diagnosis was felt to be most consistent with hyperthyroidism due to iodine excess secondary to radiological investigations using iodine based intravenous contrast agents. Amiodarone induced hyperthyroidism was considered but was thought to be unlikely as the drug was stopped a year ago.

Outcome and follow-up

Initially we started the patient on carbimazole 10 mg three times daily, but this was stopped because of impairment of liver function after 2 weeks. He was switched to propylthiouracil 100 mg three times daily and was discharged home. Runs of paroxysmal atrial fibrillation were controlled with sotalol. He re-presented to us after 2 weeks with proximal myopathy involving the lower limbs. Thyroid functions showed very little improvement, so lithium 250 mg twice daily was added. During the next couple of weeks the doses of lithium and propylthiouracil were increased. The patient showed significant clinical and biochemical improvement (FT4 21 pmol/l, FT3 2.8 pmol/l) following the addition of lithium to his treatment regimen (table 1).

Table 1.

Serial thyroid function tests

	Drug treatment regimen
	Day 0	Day 6	Day 15	Day 21	Day 30	Day 54	Day 62
Thyroid function tests	Started on carbimazole	Carbimazole 10 mg three times daily	PTU 100 mg three times daily. Switched from carbimazole because of impairment of LFT		Lithium 250 mg twice daily added	Lithium 500 mg twice daily, PTU 150 mg three times daily (dose increased on day 44)
TSH 0.5 – 4.0 mIU/l	<0.01	<0.01	<0.01	<0.01	0.01	<0.01	<0.01
FT4 10–25 pmol/l	>75	>75	>75	75	>75	31	21
FT3 3.1–5.4 pmol/l	>30.0	11.6	6.7	5.5	13.3	4.4	2.8

LFT, liver function tests; PTU, propylthiouracil; TSH, thyroid stimulating hormone.

Discussion

The average recommended intake of iodine for an adult is 150 μg/day.¹ The contrast agent mostly used in radiological imaging in South Australia is a non-ionic contrast medium containing iopromide as the active ingredient. Iopromide is a tri-iodinated, non-ionic, water soluble x ray contrast medium which comes in different strengths—that is, 150, 240, 300, 370. Each ml of 150 strength contrast contains 150 mg of iodine. A cranial CT usually uses 1–2 ml of contrast per kg of body weight. Most of the contrast is cleared by the kidneys and in patients with normal renal function iodine is excreted rapidly. Iodine induced hyperthyroidism has been reported with exposure to iodine containing drugs, povidone iodine,² dietary supplements (kelp, seaweed), and radiographic contrast dyes.

Iodine induced hyperthyroidism is a thyrotoxic condition caused by exposure to excessive iodine.³ It is also known as the Jod–Basedow phenomenon, which is a reasonably well documented condition. Iodine induced hyperthyroidism is usually described in iodine deficient areas,⁴ but very rarely it can occur in patients living in iodine sufficient areas with or without previous thyroid disease.⁵ In elderly patients sometimes it is difficult to diagnose such hyperthyroidism, as classical signs and symptoms of hyperthyroidism can be absent.^3,6,7 Clinical findings can be limited to weight loss³ and atrial fibrillation. Measuring TSH concentrations should be considered in older individuals when they undergo multiple ICC based radiological procedures.

Increased risk of atrial fibrillation has been reported, particularly in older men (over the age of 60 years) with subclinical hyperthyroidism.^8,9 Treating subclinical hyperthyroidism may benefit older individuals to prevent atrial fibrillation.⁹ Our patient was also found to have recent onset paroxysmal atrial fibrillation.

In normal subjects, adaptation to iodide intake plays a key role in maintaining the euthyroid state, even if there are variations in intake of iodine. When the thyroid gland is subjected to a sudden exposure of large iodide, biosynthesis of thyroid hormone by follicular cells decreases due to the inhibitory effect of iodide on organification of iodide and thyroid hormone formation. This self regulatory mechanism, known as the Wolff–Chaikoff effect,¹⁰ plays an important part in protecting the thyroid gland against excess biosynthesis of thyroid hormones. With continued exposure to excess iodine the thyroid gland either adapts to the increased iodine concentration or escapes from the self regulating mechanism, resulting in a rise in thyroid hormone production.¹¹

Contrast related iodine induced hyperthyroidism has mainly been reported with one radiological contrast based study, but our patient started having symptoms after seven contrast based CT scans in 8 months. One explanation, as mentioned above, could be an inhibitory effect of iodine on organification of iodine and thyroid hormone formation (Wolff–Chaikoff effect), and with repeated exposure to iodine this self regulatory mechanism was lost and led to hyperthyroidism.^11,12 Although this is mostly seen in iodine deficient regions and in individuals with underlying thyroid pathology, such as autonomous hyperfunctioning nodules, our patient did not have any previous underlying thyroid problem.

Treatment of hyperthyroidism caused by iodine excess remains a challeng as most of the drugs used in hyperthyroidism have little effect on already formed thyroid hormone and its release. Combining lithium with one of the anti-thyroid drugs has been shown to be beneficial.^13–17 The mechanism by which lithium effects the secretion of thyroid hormones is not well understood but it is known to cause impaired thyroid hormone synthesis, decreased T4 deiodination, and hypothyroidism. Although no well established study using lithium for the treatment of iodine induced hyperthyroidism has been published to date, there are case reports showing beneficial outcomes with lithium use in hyperthyroidism even before surgery when anti-thyroid drug use is limited by side effects.¹⁸ Thyroidectomy might be the only option left in those cases resistant to drugs or who develop side effects. In our patient we successfully controlled thyroid function with propylthiouracil and lithium (table 1).

Learning points

• Thyroid stimulating hormone (TSH) should be measured in all elderly patients who undergo multiple iodine containing contrast based radiological imaging, to assess for an early thyroid abnormality, thus reducing the challenges faced in treating more advanced hyperthyroidism.

• Patients with no prior thyroid abnormality can also develop hyperthyroidism due to iodine excess, and atrial fibrillation may be an isolated finding in elderly individuals.

• Lithium is beneficial in such cases due to its reasonable effect on thyroid hormone secretion.