Dietary iodine: why are so many mothers not getting enough?

Reinterpreting patterns of variation in human thyroid function: An evolutionary ecology perspective

Thyroid hormone reference intervals—used to determine normal thyroid function —currently don’t take into account many significant factors that can cause variation in thyroid hormone levels. These factors include age, sex, ethnicity, season, time of day, iodine content in the diet, socioeconomic status, stress levels, body composition, immune status, menstrual cycle phase, and overall health status. This paper shows how early life experiences as well as short term stressors may affect variation in thyroid function. These are energetic challenges to which the thyroid physiology can respond to. Our investigation shows that much variation in thyroid function is natural. It may result from a complex interplay of evolutionary, genetic, developmental, and physiological factors in response to energetic challenges in the environment, beyond what is currently considered in biomedicine. A new research agenda for thyroid health should explore the way that diversity in thyroid function has evolved as a response to different contexts people live in—like focusing on how people’s metabolisms adapt to the energetic requirements of their environments.

Two hundred million people worldwide experience some form of thyroid disorder, with women being especially at risk. However, why human thyroid function varies between populations, individuals, and across the lifespan has attracted little research to date. This limits our ability to evaluate the conditions under which patterns of variation in thyroid function are best understood as ‘normal’ or ‘pathological’. In this review, we aim to spark interest in research aimed at understanding the causes of variation in thyroid phenotypes. We start by assessing the biomedical literature on thyroid imbalance to discuss the validity of existing reference intervals for diagnosis and treatment across individuals and populations. We then propose an evolutionary ecological framework for understanding the phylogenetic, genetic, ecological, developmental, and physiological causes of normal variation in thyroid function. We build on this approach to suggest testable predictions for how environmental challenges interact with individual circumstances to influence the onset of thyroid disorders. We propose that dietary changes, ecological disruptions of co-evolutionary processes during pregnancy and with pathogens, emerging infections, and exacerbated stress responses can contribute to explaining the onset of thyroid diseases. For patients to receive the best personalized care, research into the causes of thyroid variation at multiple levels is needed.

A case-control study of urinary levels of iodine, perchlorate and thiocyanate and risk of papillary thyroid cancer

BACKGROUND

The incidence of thyroid cancer has recently increased worldwide. With the exception of radiation exposure, the effects of potential risk factors on thyroid cancer incidence remain controversial.

OBJECTIVES

The association between exposure to iodine, perchlorate, and thiocyanate and papillary thyroid cancer (PTC) incidence was evaluated and risk factors were predicted.

METHODS

A pair-matching case-control study was performed including 116 age- and sex-matched PTC cases and 116 non-PTC controls. Iodine, perchlorate, and thiocyanate concentrations in urine specimens were determined by inductively coupled plasma mass spectrometry and ultra-performance liquid chromatography-tandem mass spectrometry. The association between iodine, perchlorate, and thiocyanate urinary concentrations and PTC was evaluated using univariable conditional regression logistic analysis followed by multivariable conditional logistic regression analyses with backward stepwise selection to predict risk factors for PTC.

RESULTS

After adjusting for confounders and creatinine standardization, urinary concentrations of iodine [odds ratio (OR) = 11.01, 95% confidence interval (CI): 1.97-30.52] and perchlorate (OR = 2.27, 95% CI: 1.03-5.03) were associated with the risk of PTC, whereas urinary thiocyanate concentration showed a negative association (OR = 0.24, 95% CI: 0.09-0.65).

CONCLUSIONS

Increased exposure to iodine and perchlorate may affect PTC development, whereas high thiocyanate exposure may have a beneficial effect.

Iodine deficiency, pollutant chemicals, and the thyroid: new information on an old problem

Many women of reproductive age in the United States are marginally iodine deficient, perhaps because the salt in processed foods is not iodized. Iodine deficiency, per se, can interfere with normal brain development in their offspring; in addition, it increases vulnerability to the effects of certain environmental pollutants, such as nitrate, thiocyanate, and perchlorate. Although pregnant and lactating women should take a supplement containing adequate iodide, only about 15% do so. Such supplements, however, may not contain enough iodide and may not be labeled accurately. The American Thyroid Association recommends that pregnant and lactating women take a supplement with adequate iodide. The American Academy of Pediatrics recommends that pregnant and lactating women also avoid exposure to excess nitrate, which would usually occur from contaminated well water, and thiocyanate, which is in cigarette smoke. Perchlorate is currently a candidate for regulation as a water pollutant. The Environmental Protection Agency should proceed with appropriate regulation, and the Food and Drug Administration should address the mislabeling of the iodine content of prenatal/lactation supplements.