50 years of the ETA: "the selenium connection"

The recent celebration of the 50 years of the ETA closely coincided with that of the 200 years since the discovery and description of selenium, an essential trace element for normal thyroid gland function and thus an adjuvant in the treatment of thyroid diseases. The aim of this commentary is to briefly outline the half centennial of the ETA while also signaling important moments in the history of selenium, developments in its availability round the world, details of its connection with thyroid function and, finally, its current and projected modes of application.

The thyroid and breast cancer

PURPOSE OF REVIEW

The female predominance of diseases of the thyroid and breast makes difficult the separation of an expected association with a causal linkage. This review will examine recent reports on associations between thyroid disease and breast cancer, comparing them with previous studies, with a view to elucidating what pointers are available to suggest either a common pathogenesis or novel thyroid-related therapeutic approach, which might arise from this association.

RECENT FINDINGS

Reports on thyroid-breast cancer associations are reviewed under the following headings: breast cancer prevalence in different thyroid disorders and their effect on risk and outcome; the possible role of thyroid autoimmunity, thyroid enlargement, effect of radioactive iodine treatment, role of stable iodine, possible joint antigens sodium iodide transporter and thyroid peroxidase and thyroid-breast cancer coincidence.

SUMMARY

Current studies on thyroid and breast cancer associations confirm earlier findings of the lack of definitive evidence of a causal relationship. The predominant relationship continues to be hypothyroidism or autoimmune thyroid disease perhaps contributing to increased breast cancer risk or outcomes. However, despite many studies and the findings of meta-analyses, elucidating the mechanisms underlying the association remains elusive. At present, there is little justification for utilizing thyroid insights as a possible therapeutic intervention in breast cancer.

Regulation of iodide uptake in placental primary cultures

BACKGROUND

Maintenance of adequate iodide supply to the developing fetus is dependent not only on maternal dietary iodine intake but also on placental iodide transport. The objective of this study was to examine the effects of different pregnancy-associated hormones on the uptake of radioiodide by the placenta and to determine if iodide transporter expression is affected by hormone incubation.

METHODS

Primary cultures of placental trophoblast cells were established from placentas obtained at term from pre-labor caesarean sections. They were pre-incubated with 17β-estradiol, prolactin, oxytocin, human chorionic gonadotropin (hCG) and progesterone either singly or in combination over 12 h with (125)I uptake being measured after 6 h. RNA was isolated from placental trophoblasts and real-time RT-PCR performed using sodium iodide symporter (NIS) and pendrin (PDS) probes.

RESULTS

Significant dose response increments in (125)I uptake by trophoblast cells (p < 0.01) were observed following incubation with hCG (60% increase), oxytocin (45% increase) and prolactin (32% increase). Although progesterone (50-200 ng/ml) and 17β-estradiol (1,000-15,000 pg/ml) alone produced no significant differences in uptake, they facilitated increased uptake when combined with prolactin or oxytocin, with a combination of all four hormones producing the greatest increase (82%). Increased (125)I uptake was accompanied by corresponding increments in NIS mRNA (ratio 1.52) compared to untreated control cells. No significantly increased expression levels of PDS were observed.

CONCLUSIONS

Pregnancy-associated hormones, particularly oxytocin and hCG, have a role in promoting placental iodide uptake which may protect the fetus against iodine deficiency.

Is placental iodine content related to dietary iodine intake?

OBJECTIVE

Delivery of iodine to the foetus depends not only on maternal dietary iodine intake but also on the presence of a functioning placental transport system. A role for the placenta as an iodine storage organ has been suggested, and this study compares the iodine content of placentas from women giving birth at term in Ireland and Iran, areas with median urinary iodine of 79 and 206 μg/l respectively.

DESIGN

Placental cotyledon iodine was measured using an alkaline ashing technique with Sandell-Kolthoff kinetic colorimetry. Samples were taken from six sites from the centre and periphery of each cotyledon. Placentas (Ireland n = 58; Iran n = 45) were obtained from consecutive euthyroid women delivering at term.

RESULTS

The median placental iodine (μg/g wet weight) was significantly higher in Iranian than in Irish women (187·2 μg/g vs 34·3 μg/g; P < 0·001). The distribution of individual placental iodine values showed that values >50μg/g were found in 71·0% of Iranian and in only 21·0% of Irish samples. In Irish subjects, the relationship of placental iodine to pregnant population urinary iodine (UI) (ng/g:μg/l) was 1:2 (40:79), while in Iranians this ratio is closer to 1:1 (211:206).

CONCLUSIONS

These findings, by demonstrating an apparent ability of the placenta to store iodine in a concentration-dependent manner, suggest a hitherto undetected role for the placenta. Whether placental iodine has a role in protecting the foetus from inadequacies in maternal dietary iodine intake is as yet unknown.

Does iodine gas released from seaweed contribute to dietary iodine intake?

Thyroid hormone levels sufficient for brain development and normal metabolism require a minimal supply of iodine, mainly dietary. Living near the sea may confer advantages for iodine intake. Iodine (I(2)) gas released from seaweeds may, through respiration, supply a significant fraction of daily iodine requirements. Gaseous iodine released over seaweed beds was measured by a new gas chromatography-mass spectrometry (GC-MS)-based method and iodine intake assessed by measuring urinary iodine (UI) excretion. Urine samples were obtained from female schoolchildren living in coastal seaweed rich and low seaweed abundance and inland areas of Ireland. Median I(2) ranged 154-905 pg/L (daytime downwind), with higher values (~1,287 pg/L) on still nights, 1,145-3,132 pg/L (over seaweed). A rough estimate of daily gaseous iodine intake in coastal areas, based upon an arbitrary respiration of 10,000L, ranged from 1 to 20 μg/day. Despite this relatively low potential I(2) intake, UI in populations living near a seaweed hotspot were much higher than in lower abundance seaweed coastal or inland areas (158, 71 and 58 μg/L, respectively). Higher values >150 μg/L were observed in 45.6% of (seaweed rich), 3.6% (lower seaweed), 2.3% (inland)) supporting the hypothesis that iodine intake in coastal regions may be dependent on seaweed abundance rather than proximity to the sea. The findings do not exclude the possibility of a significant role for iodine inhalation in influencing iodine status. Despite lacking iodized salt, coastal communities in seaweed-rich areas can maintain an adequate iodine supply. This observation brings new meaning to the expression "Sea air is good for you!"

The placenta as a compensatory iodine storage organ

BACKGROUND

The production of iodine-containing thyroid hormones necessary for brain development in the fetus depends not only on maternal dietary intake but also on placental iodine transport. The optimum level of iodine nutrition during pregnancy and the proportion of the pregnant population reaching this level have previously been evaluated. Little information exists on the ability of the placenta to either accumulate or store iodine. This study aims to investigate iodine uptake and tissue iodine content within placental tissue obtained from women delivering at term.

METHODS

Samples (∼1 cm(3)) obtained from placental cotyledons (n = 19), thyroid (n = 4), and uterine myometrial (n = 4) tissue were incubated for 6 hours with (125)I in the presence and absence of potassium perchlorate. To account for variation in tissue composition, results were expressed in cpm (125)I/μg DNA.

RESULTS

Placental uptake of (125)I (375 cpm/μg DNA) was significantly higher than that of control myometrial tissue (226 cpm/μg DNA) (p < 0.05) and was ∼25% that of thyroid tissue (1702 cpm/μg DNA). Uptake of (125)I could be partially blocked in the thyroid and placenta, respectively, by potassium perchlorate (100 μM), which had no effect on uptake by myometrial tissue. Iodine content of tissue samples measured using an alkaline ashing technique with Sandell-Kolthoff colorimetry gave a mean value for total iodine of 30.4 ng/g placental tissue (range 21-50 ng/g), 1.74 ng/g myometrial tissue, and 1037 ng/g thyroid tissue.

CONCLUSIONS

Placental iodine content was only ∼3% that of the thyroid, but on the basis that neonatal iodine stores are very low and highly sensitive to fluctuations in maternal iodine supply, we postulate that placental iodine bioavailability makes a significant contribution to protection against neonatal hypothyroidism. These findings suggest that the placenta has a role not only in uptake but also in storing iodine as a possible means of protecting the fetus from inadequacies in maternal dietary iodine intake.

Perchlorate and thiocyanate exposure and thyroid function in first-trimester pregnant women

CONTEXT

Thyroid hormone, requiring adequate maternal iodine intake, is critical for fetal neurodevelopment. Perchlorate decreases thyroidal iodine uptake by competitively inhibiting the sodium/iodide symporter. It is unclear whether environmental perchlorate exposure adversely affects thyroid function in pregnant women. Thiocyanate, derived from foods and cigarette smoke, is a less potent competitive sodium/iodide symporter inhibitor than perchlorate.

OBJECTIVE

Our objective was to determine whether environmental perchlorate and/or thiocyanate exposure is associated with alterations in thyroid function in pregnancy.

DESIGN AND SETTING

We conducted a cross-sectional study at health centers in Cardiff, Wales, and Turin, Italy.

PATIENTS

During 2002-2006, 22,000 women at less than 16 wk gestation were enrolled in the Controlled Antenatal Thyroid Screening Study. Subsets of 261 hypothyroid/hypothyroxinemic and 526 euthyroid women from Turin and 374 hypothyroid/hypothyroxinemic and 480 euthyroid women from Cardiff were selected based on availability of stored urine samples and thyroid function data.

MAIN OUTCOME MEASURES

Urinary iodine, thiocyanate, and perchlorate and serum TSH, free T(4) (FT(4)), and thyroperoxidase antibody were measured.

RESULTS

Urinary iodine was low: median 98 microg/liter in Cardiff and 52 microg/liter in Turin. Urine perchlorate was detectable in all women. The median (range) urinary perchlorate concentration was 5 microg/liter (0.04-168 microg/liter) in Turin and 2 microg/liter (0.02-368 microg/liter) in Cardiff. There were no associations between urine perchlorate concentrations and serum TSH or FT(4) in the individual euthyroid or hypothyroid/hypothyroxinemic cohorts. In multivariable linear analyses, log perchlorate was not a predictor of serum FT(4) or TSH.

CONCLUSIONS

Low-level perchlorate exposure is ubiquitous but did not affect thyroid function in this cohort of iodine-deficient pregnant women.

Can neonatal TSH screening reflect trends in population iodine intake?

BACKGROUND

The distribution of neonatal blood thyroid-stimulating hormone (TSH) concentrations has been used as an index reflecting population dietary iodine intake, with higher concentrations being indicative of lower iodine intake. We examined this distribution in neonates born in Ireland, where the pregnant population has shown a recent decline in urinary iodine (UI) excretion. Our objectives were to determine if any alteration was observed in the percentage of values > 5.0 mIU/L and whether a trend in neonatal blood TSH was apparent.

METHODS

Samples drawn from the National Neonatal Screening Programme were assessed during the years 1995-2006 from winter (January n = 35,079) and summer (August n = 37,940) months, respectively, in view of the known seasonal variation in Irish dietary iodine intake.

RESULTS

Apart from the first years studied (1995-1996), the proportion of individual blood TSH values >5.0 mIU/L did not exceed 3%, a value believed to be indicative of iodine deficiency. A significant declining trend in the proportion of blood TSH >5.0 mIU/L was observed in subsequent years (p < 0.01). While excluding severe iodine deficiency, these analyses failed to detect the slight but highly significant (p < 0.001) tendency toward increasing blood TSH within the 0-5.0 mIU/L interval in the study population between 1999 and 2006, which was greater in summer than in winter months (p < 0.001).

CONCLUSIONS

These data support a link between fetal thyroid function and a fall in maternal iodine intake. While the findings of the proportion of blood TSH values >5.0 mIU/L exclude severe maternal or fetal iodine deficiency, a trend toward increasing TSH may provide an early indication of impending iodine deficiency. The findings assume greater importance in the context of declining UI reported from many developed countries even where the proportion of blood TSH values >5.0 mIU/L is <3%, thus excluding severe maternal and fetal iodine deficiency.

Changing patterns of thyroid carcinoma

BACKGROUND

To assess changing trends in histological types of thyroid cancer in an Irish hospital over the past 30 years.

METHODS

Biographical data, tumour characteristics, treatment and outcome from 190 patients with thyroid carcinoma from 1970 to 2000 were reviewed retrospectively.

RESULTS

Detailed records of 190 patients with thyroid cancer were identified with a mean age at presentation of 50 years. From 1970 to 1979 the distribution of histological types was: papillary carcinoma; 9 patients (4.7%), follicular; 17 patients (8.9%), anaplastic; 9 patients (4.7%), medullary; 1 patient (0.5%) and lymphoma; 1 patient (0.5%). From 1980 to 1989 papillary carcinoma accounted for 32 patients (16.8%), follicular; 14 patients (7.3%), anaplastic; 13 patients (6.8%), medullary; 7 patients (3.7%) and lymphoma; 5 patients (2.6%). From 1990 to 1999 papillary cancer accounted for 48 patients (25.2%), follicular; 14 patients (7.3%), anaplastic; 8 patients (4.2%), medullary; 7 patients (3.7%) and lymphoma; 5 patients (2.6%). Survival rates were significantly better for those aged less than 45 years (P < 0.0001), female sex (P < 0.01) and those with papillary carcinoma (P < 0.01).

CONCLUSIONS

This study demonstrated a significant increase in the incidence of papillary carcinoma. This may be related to increasing dietary iodine intake and may be significant as papillary carcinoma is associated with a more favourable prognosis.

Short-term changes in maternal and neonatal urinary iodine excretion

Investigation of maternal urinary iodine (UI) excretion in the immediate antenatal and early postpartum periods showed a precipitous fall in median values from 93 microg/L antenatally to 36 microg/L at delivery subsequently rising to 49 microg/L and 63 microg/L at days 3 and 10 postpartum respectively. The fate of ingested iodine not appearing in the maternal urine is unknown but measurement of UI in babies born to nursing mothers suggested transfer from the mother with median neonatal values of 117 and 159 microg/L being recorded at days 3 and 10. While maternal UI seemed to relatively unaffected by breast feeding, median UI from breast feeding babies (148 microg/L) was significantly greater than in those bottle feeding (50 microg/L). This was also reflected by the finding that no breast feeding baby had a UI values < 50 microg/L in comparison to 50% of bottle feeders. The depressed values in mothers and relatively high values in their infants could present a false picture and suggest the need to defer any investigations of iodine status at this time. The findings do however suggest a need for further investigations aimed at determining the fate of iodine ingested perinatally and its possible physiological significance in maintaining thyroid status in the mother and neonate.

Iodine intake in pregnancy in Ireland — A cause for concern?

BACKGROUND

Adequate dietary iodine intake is necessary to maintain maternal thyroid function at a level permitting normal neuropsychological development of the foetus.

AIMS AND METHODS

To determine dietary iodine status by measuring urinary iodine excretion (UIE), proportional to dietary intake, in Irish mothers during the first trimester of pregnancy.

RESULTS

Median UIE showed seasonal variations, being lower in summer than in winter. The median values in pregnant women were, summer 45microg/l, winter 68microg/l. Equivalent values for controls were 43 and 91microg/l respectively. UIE required to achieve WHO recommended daily iodine intakes would be 120-180microg/l. In the Irish subjects UIE values suggestive of iodine deficiency (<50microg/l) were observed in 55% of pregnant women tested in summer and 23% in winter. Dairy milk iodine, a major dietary iodine source, showed similar variation.

CONCLUSIONS

While there is as yet no available evidence of widespread thyroid hypofunction in the Irish obstetric population, the findings are a cause of concern, which if confirmed by a more comprehensive investigation, may indicate the need for iodine prophylaxis.

Thyroglobulin autoantibodies in iodized subjects: relationship between epitope specificities and longitudinal antibody activity

INTRODUCTION

We previously reported a high thyroglobulin autoantibodies (TgAb) prevalence in healthy Sri Lankans after iodine supplementation. In the present study 58 TgAb-positive schoolgirls were followed up after 5 years of continued iodination. The objectives were: (1) to observe the longitudinal profile of TgAb epitope specificities and (2) to examine the relationship between these specificities and the course of thyroid autoimmunity in this population.

METHODS

Paired subjects' sera (at onset and at 5-year follow-up) were tested for TgAb, thyroid peroxidase antibody (TPOAb), and TgAb epitope-specificity. Epitope reactivity was determined by employing a panel of 10 murine monoclonal antibodies (Tg-mAbs) directed against 6 Tg antigenic clusters (I-VI) in competitive enzyme-linked immunosorbent assay (ELISA) reactions with test sera.

RESULTS

The overall pattern of epitope recognition in individual subject's sera remained preserved over the time period. Nine subjects showed restricted specificities while majority of the subjects were broadly heterogeneous. At follow-up, median TgAb concentration in the restricted group was higher than in the unrestricted (1650 versus 110 kIU/L; p < 0.005). Epitope specificity was a stronger determinant of TgAb persistence than the height of the initial TgAb response or the TPOAb status of subjects.

CONCLUSION

Tg epitope reactivity pattern in iodised populations may identify subjects at greater risk of developing autoimmune thyroid disease (AITD).

Sequential studies on thyroid antibodies during pregnancy

Thyroid antibodies were measured sequentially in 25 pregnant women from a Sri Lankan population. A high prevalence of antithyroid antibodies, particularly antithyroglobulin antibodies (TgAb) had previously been demonstrated in female schoolchildren drawn from this population. In the present study TgAb were detected in 36.8% of nonpregnant controls while thyroid peroxidase antibody (TPOAb) positivity was present in 26.3%. The prevalence of both antibodies in the pregnancy study group showed a progressive decline compared to nonpregnant controls throughout gestation becoming undetectable in the third trimester. The results are consistent with an immunosuppressive effect of pregnancy in a population in whom high thyroid autoantibody titers may have resulted from a recent salt iodization program.

Thyroglobulin epitope recognition in a post iodine-supplemented Sri Lankan population

OBJECTIVE

We previously reported a high prevalence of raised thyroglobulin autoantibodies (TgAb) in apparently healthy Sri Lankan schoolgirls following salt iodination. To characterize these antibodies further we determined the epitopes on thyroglobulin (Tg) with which they react and compared these with serum obtained from both healthy subjects and established autoimmune thyroid disease (AITD) patients from the UK. To extend our study to a wider population within Sri Lanka, we in addition determined the epitopes recognized by a group of AITD patients selected from a thyroid clinic in Sri Lanka, as well as apparently healthy female Sri Lankan tea workers of distinct ethnicity from the schoolgirls and AITD patients.

DESIGN

Sri Lankan schoolgirls (n = 282) and adult female tea estate workers (n = 208) were examined for thyroid autoimmune markers. Sera with high TgAb (> 98 kIU/l) were selected from these two groups (n = 36 and 45, respectively) to study epitope-binding patterns. We also examined the sera from 16 AITD patients attending a thyroid clinic in Colombo, 16 patients with AITD from the thyroid clinic at the University Hospital of Wales and 16 sera from healthy control UK women with no evidence of thyroid disease. To determine the epitopes on Tg recognized by the subjects' TgAb, we employed a panel of Tg mouse monoclonal antibodies labelled with alkaline phosphatase in a competitive enzyme-linked immunosorbent assay reaction with the subjects' serum.

RESULTS AND CONCLUSIONS

A majority of the Sri Lankan schoolgirls did not react with the immunodominant epitopes and did not differ significantly from healthy subjects from the UK in their Tg epitope recognition pattern. On the other hand, tea estate workers and Sri Lankan AITD patients recognized typical autoimmune thyroid disease epitopes and, in addition, recognized a separate cluster not previously associated with either the autoimmune state or the healthy state. The significance of this cluster requires further clarification.

Evolution of thyroid autoimmunity during iodine prophylaxis--the Sri Lankan experience

OBJECTIVE

To study the evolution of thyroid autoimmunity, in relation to the change in goitre prevalence, during 3 Years of iodine prophylaxis in Sri Lanka.

METHODS

Two groups of Sri Lankan schoolgirls between the ages of 10.8 and 17.5 Years were studied in 1998 (401 girls) and 2001 (282 girls). A prospective study was performed in 42 schoolgirls who were thyroid autoantibody (Ab)-positive (+ve) in 1998. Anthropometric measures, urinary iodine excretion (UIE), thyroid Volume, free thyroxine, free tri-iodothyronine, TSH, and thyroglobulin (Tg) and thyroid peroxidase (TPO) Ab were evaluated in all 683 girls.

RESULTS

Goitre prevalence was significantly lower in 2001 compared with 1998 related to age (2.9% compared with 20.2%) and body surface area (11.6% compared with 40.8%), although UIE was unchanged. Prevalence of thyroid Ab in 2001 was also lower (23.4% compared with 49.9%); among those with the Ab, 34.8% had TgAb alone and 46.9% had a combination of TgAb+TPOAb, compared with 82.0% TgAb alone in 1998. In 2001, subclinical hypothyroidism was more frequent in Ab+ve (6.3%) than Ab-negative girls (1.0%). A cohort of 42 Ab+ve schoolgirls in 1998 (34 with TgAb alone, eight with TgAb+TPOAb) were evaluated again in 2001. Only 10 of them (23.8%) remained Ab+ve (mostly TPOAb+/-TgAb) in 2001.

CONCLUSIONS

This study demonstrates that: (1) in 2001, goitre prevalence and thyroid autoimmunity rates were significantly lower than in 1998; (2) the pattern of thyroid Ab was different in the two surveys; (3) in 2001 alone, the occurrence of hypothyroidism was correlated with the presence of thyroid autoimmunity. These results indicate an evolution of thyroid autoimmune markers during the course of iodine prophylaxis, which has not been described before.

The thyroid, iodine and breast cancer

A renewal of the search for a link between breast cancer and thyroid disease has once again demonstrated an increased prevalence of autoimmune thyroid disease in patients with breast cancer. This is the most recent of many studies showing an association between a variety of thyroid disorders and breast cancer. Such an association is not surprising as both diseases are female predominant with a similar postmenopausal peak incidence. The significance of the presence of thyroid autoantibodies, particularly thyroid peroxidase antibodies, in serum from patients with breast cancer is unknown, but it has been suggested that antibody positivity is associated with better prognosis. One area in which thyroid and breast functions overlap is in the uptake and utilization of dietary iodide. Experimental findings showing the ability of iodine or iodine-rich seaweed to inhibit breast tumour development is supported by the relatively low rate of breast cancer in Japanese women who consume a diet containing iodine-rich seaweed. However, there is as yet no direct evidence that iodine, iodinated compounds, or a combination of iodine and selenium is the antimammary carcinogenic element in the Japanese diet. It remains to be resolved whether the perceived breast cancer-thyroid disease relationship is thyroid or iodine related or, in the case of thyroid autoantibodies, is the consequence of an immune response to the carcinoma. Is this response breast specific and does it relate to iodine status? These and many other questions await resolution before a definitive role in the natural history of breast carcinoma can be assigned to the thyroid.

Role of iodine in antioxidant defence in thyroid and breast disease

The role played in thyroid hormonogenesis by iodide oxidation to iodine (organification) is well established. Iodine deficiency may produce conditions of oxidative stress with high TSH producing a level of H_2O_2, which because of lack of iodide is not being used to form thyroid hormones. The cytotoxic actions of excess iodide in thyroid cells may depend on the formation of free radicals and can be attributed to both necrotic and apoptotic mechanisms with necrosis predominating in goiter development and apoptosis during iodide induced involution. These cytotoxic effects appear to depend on the status of antioxidative enzymes and may only be evident in conditions of selenium deficiency where the activity of selenium containing antioxidative enzymes is impaired. Less compelling evidence exists of a role for iodide as an antioxidant in the breast. However the Japanese experience may indicate a protective effect against breast cancer for an iodine rich seaweed containing diet. Similarly thyroid autoimmunity may also be associated with improved prognosis. Whether this phenomenon is breast specific and its possible relationship to iodine or selenium status awaits resolution.