Yes, the TSH level provides the information you need to manage primary hypothyroidism.
Soamsiri Niwattisaiwong, MD
Department of Endocrinology, Sanford Health, Fargo, ND
Kenneth D. Burman, MD
Endocrinology Section, MedStar Washington Hospital Center, Washington, DC
Melissa Li-Ng, MD
Department of Endocrinology, Diabetes, and Metabolism, Cleveland Clinic
Address: Melissa Li-Ng, MD, Department of Endocrinology, Diabetes and Metabolism, F20, Cleveland Clinic, 9500 Euclid Avenue, Cleveland, OH 44195; firstname.lastname@example.org
Iodine is crucial for thyroid hormone synthesis and fetal neurodevelopment. Major dietary sources of iodine in the United States are dairy products and iodized salt. Potential consequences of iodine deficiency are goiter, hypothyroidism, cretinism, and impaired cognitive development. Although iodine status in the United States is considered sufficient at the population level, intake varies widely across the population, and the percentage of women of childbearing age with iodine deficiency is increasing. Physicians should be aware of the risks of iodine deficiency and the indications for iodine supplementation, especially in women who are pregnant or lactating.
A 65-year-old woman is found to have a goiter. She is clinically euthyroid. She is a strict vegan and only uses noniodized Himalayan salt for cooking. Her thyroid gland is diffusely enlarged with no nodules. The estimated weight of the thyroid gland is 50 g (normal 10–20 g) based on ultrasonography. Her thyroid-stimulating hormone (TSH) level is 2.95 mU/L (reference range 0.5–5 mU/L), and her free thyroxine level is 0.8 ng/dL (0.7–1.8 ng/dL). Testing for TSH receptor antibody is negative. Her 24-hour urine iodine is undetectable (urine iodine concentration < 10 μg/L with urine volume 3,175 mL). What may be the cause of her goiter?
Iodine is an essential element needed for the production of thyroid hormone, which controls metabolism and plays a major role in fetal neurodevelopment. Its ionized form is called iodide. Iodine deficiency results in impairment of thyroid hormone synthesis and may lead to several undesirable consequences. Physicians should be aware of the risks iodine deficiency poses, especially during pregnancy, and should be familiar with approaches to testing and current indications for iodine supplementation.
The major environmental source of iodine is the ocean. Elemental iodine in the ocean volatilizes into the atmosphere and returns to the soil by rain. The effects of glaciation, flooding, and leaching into soil have resulted in the variable geographic distribution of iodine. Mountainous areas (eg, the Alps, Andes, Himalayas) and areas with frequent flooding typically have iodine-deficient soil due to slow iodine cycling.1 Seafood is a good source of iodine because marine plants and animals concentrate iodine from seawater. The iodine content of other foods varies widely, depending on the source and any additives.
In the United States, the major sources of dietary iodine are dairy products (due to livestock iodine supplements and use of iodophors for cleaning milk udders) and iodized salt.1,2 Seafood contains a higher amount of iodine by weight than dairy products but is consumed far less than dairy.3,4 Further, the iodine content of milk can range from 88 to 168 μg per 250 mL (about 1 cup), depending on the product manufacturer. Also, iodine content is often omitted from the food label. Even if it is reported, the package labeling may not accurately predict the iodine content.5
Less common sources of iodine are radiographic contrast, bread with iodate dough conditioners, red food coloring (erythrosine), and drugs such as amiodarone.1
Using iodized salt is an effective and stable way to ensure adequate iodine intake. In the United States, only table salt is iodized, and the salt typically used in processed food has only minimal iodine content.6 Nearly 70% of the salt we ingest is from processed food. Table salt provides only 15% of dietary salt intake, and only 70% of consumers choose iodized salt for home cooking.7
Daily requirements of iodine suggested by the World Health Organization (WHO) and by the US Institute of Medicine are in the range of 90 to 150 µg/day.8,9 The iodine requirement is higher in pregnancy (220–250 µg/day) because of increased maternal thyroid hormone production required to maintain euthyroidism and increased renal iodine clearance, and it is even higher in lactating women (250–290 µg/day).
Since the establishment of universal salt iodization programs under the influence of the WHO and the International Council for Control of Iodine Deficiency Disorders (ICCIDD) in 1990, global iodine status has continued to improve. Yet only 70% of households worldwide currently have access to adequately iodized salt, because many countries lack a national program for iodine supplementation. The population of the United States was historically iodine-deficient, but since the introduction of salt iodization in the 1920s, the iodine status in the United States has been considered adequate.1
The WHO defines iodine status for a population by the median spot urinary iodine concentration. Because a urinary iodine concentration of 100 μg/L represents an iodine intake of about 150 μg/day, the WHO uses a median urinary iodine concentration of 100 to 199 μg/L to define adequate iodine intake for a nonpregnant population.9
The National Health and Nutrition Examination Survey (NHANES) found that the median urinary iodine concentration decreased by more than 50% from the 1970s to the 1990s, indicating declining iodine status in the US population.2 Of particular concern, the percentage of women of childbearing age with moderate iodine deficiency increased from 4% to 15% over this period.2 Still, the NHANES survey in 2009–2010 indicated that the overall US population is still iodine-sufficient (median urinary iodine concentration 144 μg/L).10 The decline in the US iodine status may be due to reduction of iodine content in dairy products, increased use of noniodized salt by the food industry, and recommendations to avoid salt for blood pressure control.
Although US iodine status has been considered generally adequate, iodine intake varies greatly across the population. Vegans tend to have iodine-deficient diets, while kelp consumers may have excessive iodine intake.11 Individuals with lactose intolerance are at risk of iodine deficiency, given that dairy products are a major source of iodine in the United States. Physicians should be aware of these risk factors for iodine deficiency.
It is crucial to maintain euthyroidism during pregnancy. In early gestation, maternal thyroid hormone production increases 50% due to an increase in thyroid-binding globulin and stimulation by human chorionic gonadotropin. The glomerular filtration rate increases by 30% to 50% during pregnancy, thus increasing renal iodine clearance. Fetal thyroid hormone production increases during the second half of pregnancy, further contributing to increased maternal iodine requirements because iodine readily crosses the placenta.12
Women with sufficient iodine intake before and during pregnancy generally have adequate intrathyroidal iodine storage and can adapt to the increased demand for thyroid hormone throughout gestation. But in the setting of even mild iodine deficiency, total body iodine stores decline gradually from the first to third trimester of pregnancy.13
The fetal thyroid gland does not begin to concentrate iodine until 10 to 12 weeks of gestation and is not controlled by TSH until the full development of the pituitary-portal vascular system at 20 weeks of gestation.12 Therefore, the fetus relies on maternal thyroid hormone during this critical stage of neurodevelopment. Thyroid hormone is essential for oligodendrocyte differentiation and myelin distribution14 as well as fetal neuronal proliferation and migration in the first and second trimesters. Iodine deficiency leading to maternal hypothyroidism can result in irreversible fetal brain damage.
Because of the greater requirement during pregnancy, the WHO recommends using a median urinary iodine concentration of 150 to 249 μg/L to define a population that has no iodine deficiency.9 The NHANES data from 2007 to 2010 showed that pregnant US women were mildly iodine-deficient (median urinary iodine concentration 135 μg/L),10 and the National Children’s Study of 501 pregnant US women during the third trimester in 2009 to 2010 showed they had adequate iodine intake (median urinary iodine concentration 167 μg/L). Interestingly, pregnant non-Hispanic blacks were the only ethnic group with a median urinary iodine concentration less than 150 μg/L, suggesting that race or ethnicity is a predictor of iodine status in pregnant women.10
Yes, the TSH level provides the information you need to manage primary hypothyroidism.