Thyroxine (T4) and triiodothyronine (T3): effects of iodine on the serum concentrations and disposal rates in subjects from an endemic goiter area

Sustained Elimination of Iodine Deficiency Within the Third Decade After Compulsory Iodine Supplementation Policy in the South of Iran: A Population-Based Cross-Sectional Study

Objectives

Recently, some studies in Iran have shown mild to moderate iodine concentrations in adult and pregnant women populations despite iodine sufficiency in children. This study aimed to evaluate the urine iodine status and salt intake among adult households in the city of Sadra, Fars province in southern Iran, and to assess its possible influencing factors.

Method

Participant households for this cross-sectional study were selected using randomized cluster sampling in the city of Sadra, Fars province, southern Iran from 1 February, 2021 to 30 November, 2021. Two subjects >18 y of age from each household were invited. Ninety-two subjects (24 men, 68 women) were enrolled. The participants were asked to collect their 24 h urine. They were then examined for thyroid disorders and subjected to thyroid ultrasonography and thyroid function tests. Urine samples were tested for iodine, sodium, and creatinine concentrations. Household salt intake was also estimated.

Results

Median urine iodine content (UIC) in the participants was 175 (IQR: 117, 250) μg/L, whereas the median salt consumption per person per day was 9.6 (IQR, 7.3-14.5) g. Sex, methods of salt storage, presence of goiter or thyroid nodules, the addition of salt in the cooking stage, and subclinical hypothyroidism had no effect on UIC, whereas individuals with hypertension and lower education had significantly lower iodine concentrations. UIC had a significant positive correlation with urine sodium and thyroid stimulating hormones (TSH) concentrations (P < 0.001, 0.046) and a negative correlation with thyroid volume and T4 (P = 0.029, 0.018).

Conclusion

Iodine status in the adult population of Sadra city was categorized as sufficient, although the iodine concentrations reported in Tehran were insufficient. The contributing factor can be higher salt consumption or possible higher environmental iodine concentrations in Sadra city than Tehran.

Urinary iodine, thyroid function, and thyroglobulin as biomarkers of iodine status

The accurate assessment of population iodine status is necessary to inform public health policies and clinical research on iodine nutrition, particularly the role of iodine adequacy in normal neurodevelopment. Urinary iodine concentration (UIC) directly reflects dietary iodine intake and is the most common indicator used worldwide to assess population iodine status. The CDC established the Ensuring the Quality of Iodine Procedures program in 2001 to provide laboratories that measure urinary iodine with an independent assessment of their analytic performance; this program fosters improvement in the assessment of UIC. Clinical laboratory tests of thyroid function (including serum concentrations of the pituitary hormone thyrotropin and the thyroid hormones thyroxine and triiodothyronine) are sometimes used as indicators of iodine status, although such use is often problematic. Even in severely iodine-deficient regions, there is a great deal of intraindividual variation in the ability of the thyroid to adapt. In most settings and in most population subgroups other than newborns, thyroid function tests are not considered sensitive indicators of population iodine status. However, the thyroid-derived protein thyroglobulin is increasingly being used for this purpose. Thyroglobulin can be measured in either serum or dried blood spot (DBS) samples. The use of DBS samples is advantageous in resource-poor regions. Improved methodologies for ascertaining maternal iodine status are needed to facilitate research on developmental correlates of iodine status. Thyroglobulin may prove to be a useful biomarker for both maternal and neonatal iodine status, but validated assay-specific reference ranges are needed for the determination of iodine sufficiency in both pregnant women and neonates, and trimester-specific ranges are possibly needed for pregnant women. UIC is currently a well-validated population biomarker, but individual biomarkers that could be used for research, patient care, and public health are lacking.

Iodine deficiency and thyroid disorders

Iodine deficiency early in life impairs cognition and growth, but iodine status is also a key determinant of thyroid disorders in adults. Severe iodine deficiency causes goitre and hypothyroidism because, despite an increase in thyroid activity to maximise iodine uptake and recycling in this setting, iodine concentrations are still too low to enable production of thyroid hormone. In mild-to-moderate iodine deficiency, increased thyroid activity can compensate for low iodine intake and maintain euthyroidism in most individuals, but at a price: chronic thyroid stimulation results in an increase in the prevalence of toxic nodular goitre and hyperthyroidism in populations. This high prevalence of nodular autonomy usually results in a further increase in the prevalence of hyperthyroidism if iodine intake is subsequently increased by salt iodisation. However, this increase is transient because iodine sufficiency normalises thyroid activity which, in the long term, reduces nodular autonomy. Increased iodine intake in an iodine-deficient population is associated with a small increase in the prevalence of subclinical hypothyroidism and thyroid autoimmunity; whether these increases are also transient is unclear. Variations in population iodine intake do not affect risk for Graves' disease or thyroid cancer, but correction of iodine deficiency might shift thyroid cancer subtypes toward less malignant forms. Thus, optimisation of population iodine intake is an important component of preventive health care to reduce the prevalence of thyroid disorders.

The thyroid in ulcerative colitis and Crohn's disease. V. Triiodothyronine. Effect of corticosteroids and influence of severe disease

The concentrations of triiodothyronine (T3), thyroxine (T4) and thyroxine binding globulin (TBG) have been measured in serum of 20 patients with ulcerative colitis or Crohn's disease. The patient group was compared with 20 healthy control subjects matched for sex and age. The concentration of T3 and T4 were similar in the two groups but TBG in serum was higher in the patient group, mainly due to the high TBG levels in the female patients. The concentration of T3 in serum was lower in the severely ill patients than in those who were mildly-moderately ill, while T4 and TBG were not affected by the severity of the disease. The concentration of T3 was lower in the corticosteroid-treated patients than in those who did not have such treatment, just like the TBG level. However, TBG was not subnormal in the corticosteroid-treated patients, whereas the serum concentration of T3 was. T4 in serum was not affected by treatment with corticosteroids. These findings indicate that the metabolism of T4 and T3 are influenced differently by corticosteroids.

Nutritional Epidemiology and Thyroid Hormone Metabolism

Severe iodine deficiency was the main cause of endemic goiter and cretinism. Most of the previously iodine-deficient areas are now supplemented, mainly with iodized salt. The geographical distribution of severe endemic areas has been progressively reduced, and at present, approximately 200 million people living in remote places are still at risk of severe iodine deficiency. International public health programs should be focused first on reaching these populations, and second on auditing and monitoring the operational work of supplementation programs. This second point is essential to prevent iodine-induced hyperthyroidism or interruptions of iodine supplement distribution, which could be catastrophic for the fetus and the young infant. Echography brings a complementary tool to clinical assessment of goiter by palpation. Inductively coupled plasma-mass spectrometry brings at least a definitive gold standard for iodine measurement and thyroid hormone measurement. Thiocyanate overload has been clearly documented as a goitrogen in Central Africa, and when associated with selenium deficiency, it may be included as risk factor for endemic myxedematous cretinism. Variable exposure to different environmental risk factors is likely the explanation of the variable distribution of two types of endemic cretinism (neurological and myxedematous), and the clinical overlap of the pathogeny of both syndromes is more important than previously described. It is possible that Kashin-Beck osteoarthropathy is another evanescent endemic disease that will disappear with the correction of iodine deficiency.

Transient thyrotoxicosis in endemic goitre patients following exposure to a normal iodine intake

Twenty-three endemic goitrous subjects (goitre grade: III and IV) that were living in a chronic iodine-deficient area (iodine intake: less than 40 micrograms I/d) were submitted to clinical and laboratory evaluation within 3-8 weeks of arrival at the metropolitan area of São Paulo, where daily iodine intake is estimated to be 150-200 micrograms I/d. Eight patients developed a mild thyrotoxic state (T4 = 14.7 +/- 2.3 micrograms/dl, T3 = 279 +/- 55 ng/dl, no TSH response to TRH). Five additional subjects, although euthyroid, had a blunted TSH-response to TRH, and the remaining ten patients were euthyroid and had a normal TSH response to TRH. Thyrotoxicosis was associated with larger goitres (mean thyroid weight: 133 +/- 46 g), with high thyroid uptake of RAI (mean 24 h 131I uptake: 40 +/- 15%) but not with increased urinary iodine excretion. Serum Tg levels were more elevated in the first two groups of patients (respectively, geometric means 68 and 72 ng/ml), than in the euthyroid, TRH-responsive group (52 ng/ml). Thyrotoxicosis resolved spontaneously after three to six months without the need for any specific medication. It was concluded that a relatively small and normal iodide intake due to regular consumption of iodized salt and industrialized foods may induce a transient form of thyrotoxicosis in endemic goitre patients arriving into urban areas.

Nutritional status and endemic goitre

Biochemical indices of thyroid function and serum-retinol levels in adult goitrous patients from an endemic area in Senegal were compared with those of non-goitrous controls from an area in which goitre was not endemic. The findings (reduced thyroid-binding proteins and reduced serum-retionol levels) could be accounted for by protein malnutrition. They also showed that in goitrous and malnourished patients, the feedback regulation of thyrotropic hormone is governed more by thyroxine than by triiodothyronine and that in such patients serum thyroxine is better than serum triiodothyronine as an indicator of thyroid function.

The toxic effect of small iodine supplements in patients with autonomous thyroid nodules

In sixteen cases of toxic adenoma of the thyroid (autonomous hot nodule with complete suppression of the surrounding normal parenchyma) potassium iodide was given in doses of 100 microgram/day for one week, 200 microgram/day for another and 400 microgram/day for a third week. There was a progressive increase in the serum T4 level. Serum T3 also increased, although this was significant only after the first week. Serum TSH was undetectable throughout the entire period of the study. This metabolic pattern is different from the response seen in cases of nontoxic endemic goitre, where small iodine supplements induce an increase in serum T4 but a decrease in serum T3. Furthermore, the present results may explain the phenomenon of iodine-induced or iodine-precipitated hyperthyroidism (Jod-Basedow) when patients with autonomous thyroid are presented with a high iodine intake. In contrast to the results obtained with small iodide doses, two other cases treated with large pharmacological doses of iodide showed a decrease in both serum T4 and serum T3. It is concluded that the physician should be aware of the possibility of precipitating or aggravating thyrotoxicosis in patients with an autonomous hot nodule by increasing their intake of iodine.

[Thyroidal autoregulation in nontoxic goiter patients: normalization of the T3/T4 ratio in serum by treatment with potassium iodide (author's transl)]

In iodine deficiency areas, an increased T3/T4 ratio in serum was reported earlier. In this study the influence of potassium iodide on the T3/T4 ratio was investigated. Treatment with 200 mug KI per day for only 4 weeks normalized the initially elevated T3/T4 ratio in 16 nontoxic goiter patients (24.6 plus or minus 9.9 times 10-3, mean plus or minus S.D., before, respectively 18.9 plus or minus 5.5 times 10-3 after KI treatment, p smaller than 0.0025). The mean basal TSH levels (1.42 plus or minus 0.68 before, respectively 1.29 plus or minus 0.52 mugU/ml after KI), and the TSH increase 30 min after 200 mug TRH i.v. (5.54 plus or minus 4.22 and 6.50 plus or minus 5.09 mugU/ml, respectively) did not change significantly. Since the normalization of the T3/T4 ratio in nontoxic goiter patients was independent from changes in the TSH levels, this effect of KI obviously represents an example of thyroidal autoregulation.