Effect of Iodine Restriction on Thyroid Function in Patients With Primary Hypothyroidism

Kanji Kasagi, Masahiro Iwata, Takashi Misaki, Junji Konishi

Disclosures

Thyroid. 2003;13(6) 

In This Article

Abstract and Introduction

Dietary iodine intake in Japan varies from as little as 0.1 mg/day to as much as 20 mg/day. The present study was undertaken to assess the frequency of iodine-induced reversible hypothyroidism in patients diagnosed as having primary hypothyroidism, and to clarify the clinical backgrounds responsible for the spontaneous recovery of thyroid functions. Thirty-three consecutive hypothyroid patients (25 women and eight men) with a median age of 52 years (range, 21-77 years) without a history of destructive thyroiditis within 1 year were asked to refrain from taking any iodine-containing drugs and foods such as seaweed products for 1-2 months. The median serum thyrotropin (TSH) level, which was initially 21.9 mU/L (range, 5.4-285 mU/L), was reduced to 5.3 mU/L (range, 0.9-52.3 mU/L) after iodine restriction. Twenty-one patients (63.6%) showed a decrease in serum TSH by >50% and to <10 mU/L. Eleven patients (33.3%) became euthyroid with TSH levels within the normal range (0.3-3.9 mU/L). The ratios of TSH after iodine restriction to TSH before iodine restriction (aTSH/bTSH) did not correlate significantly with titers of anti-thyroid peroxidase antibody and anti-thyroglobulin antibody or echogenicity on ultrasonography, but correlated inversely with 99mTc uptake (r = 0.600, p < 0.001). Serum non-hormonal iodine levels, although not correlated significantly with aTSH/bTSH values, were significantly higher in the 21 patients with reversible hypothyroidism than in the remaining 12 patients. TSH binding inhibitor immunoglobulin was negative in all except one weakly positive case. In conclusion, (1) primary hypothyroidism was recovered following iodine restriction in more than half of the patients, and (2) the reversibility of hypothyroidism was not significantly associated with Hashimoto's thyroiditis but with increased 99mTc uptake and elevated non-hormonal iodine levels.

Iodine-induced hypothyroidism is not rare, at least in Japan, where seaweed is habitually ingested.[1,2,3,4,5] Dietary iodine intake varies from as little as 0.1 mg/day to as much as 20 mg/day. Young Japanese prefer to eat iodine-poor westernized dishes, whereas old or middle-aged Japanese often eat seaweed, presumably because of habit. Recently, intake of seaweed, a low-calorie food containing sufficient calcium, potassium, iron, and vegetable fibers, has been recommended for prevention of future development of arteriosclerotic diseases such as ischemic heart diseases, cerebrovascular diseases, disorders of lipid metabolism, diabetes mellitus, hypertension, and obesity, as well as the occurrence of osteoporosis and iron-deficiency anemia.

From the clinical point of view, it is important to differentiate hypothyroid patients who may recover spontaneously after iodine restriction from those who may not. Although Hashimoto's thyroiditis[6,7] is thought to be the underlying disease, anti-thyroid antibodies are not detected in some patients with reversible hypothyroidism.[2,3,4,5,8] The present study was undertaken to assess the frequency of iodine-induced reversible hypothyroidism in patients diagnosed as having primary hypothyroidism, and to clarify the clinical backgrounds responsible for the spontaneous recovery of hypothyroidism. In this study, anti-thyroid antibodies such as anti-thyroglobulin antibody (TgAb), anti-thyroid peroxidase antibody (TPOAb), and TSH binding inhibitor immunoglobulin (TBII), sonographic findings, 99mTc thyroidal uptake values, and serum non-hormonal iodine levels were determined in patients with reversible hypothyroidism and compared with those in patients with irreversible hypothyroidism.

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