Thyroid stimulating hormone (TSH) autoregulation reduces variation in the TSH response to thyroid hormones

Food addiction, hormones and blood biomarkers in humans: A systematic literature review

BACKGROUND

Food addiction may play a role in rising obesity rates in connection with obesogenic environments and processed food availability, however the concept of food addiction remains controversial. While animal studies show evidence for addictive processes in relation to processed foods, most human studies are psychologically focussed and there is a need to better understand evidence for biological mechanisms of food addiction in humans. Several key hormones are implicated in models of food addiction, due to their key roles in feeding, energy metabolism, stress and addictive behaviours. This systematic literature review examines evidence for relationships between food addiction, hormones and other blood biomarkers.

METHODS

A series of literature searches was performed in Scopus, PsychInfo, MedLine, ProQuest, CINAHL and Web of Science. A total of 3111 articles were found, of which 1045 were duplicates. Articles were included if they contained a psychometric measurement of food addiction, such as the Yale Food Addiction Scale, as well as addressed the association between FA and hormones or blood biomarkers in humans. Articles were assessed for eligibility by two independent reviewers.

RESULTS

Sixteen studies were identified that examined relationships between food addiction and blood biomarkers, published between 2015 and 2021. Significant findings were reported for leptin, ghrelin, cortisol, insulin and glucose, oxytocin, cholesterol, plasma dopamine, thyroid stimulating hormone (TSH), haemoglobin A1c (HbA1c), triglyceride (TG), amylin, tumour necrosis factor alpha (TNF- α) and cholecystokinin (CCK). Methodological issues included small sample sizes and variation in obesity status, sex and mental health-related comorbidities. Due to methodological limitations, definite connections between FA, hormones and other blood biomarkers cannot yet be determined.

CONCLUSION

This systematic review identified preliminary evidence linking FA symptoms to hormones and other blood biomarkers related to feeding, addiction, and stress. However, due to the small number of studies and methodological limitations, further research is needed to evaluate biopsychosocial models of FA and to resolve controversies.

Physiological linkage of thyroid and pituitary sensitivities

OBJECTIVES

The sensitivities of the pituitary to thyroxine feedback, and the thyroid to thyrotropin stimulation determine the free thyroxine /thyrotropin feedback loop and can be described mathematically by two curves. It is not well understood how the two curves combine in a healthy population with normal thyroid function to express the individual balance points that are observed. This study was directed at this issue testing the possibilities of random combination and directed linkage between the two curves.

METHODS

We reverse-engineered two sets of population data, on the assumption of independent combinations of thyroid and pituitary sensitivities, to obtain estimates of the curve describing thyroid sensitivity. Sensitivity studies were performed.

RESULTS

No analysis resulted in a physiologically feasible estimate of the curve describing thyroid sensitivity. There was evidence of linkage of the two curves in terms of their combination throughout the normal range. Thyroid response curves reflecting a low free thyroxine response to thyrotropin tended to be combined in individuals with thyrotropin curves reflecting a high thyrotropin response to free thyroxine, and vice versa.

CONCLUSIONS

Thyroid and pituitary sensitivities are linked, being combined in individuals in a non-random directed pattern. Direct mutual interaction may contribute to this linkage. This linkage precludes the derivation of the curves describing these sensitivities from population data of the free thyroxine and thyrotropin relationship and complicates their derivation by physiological experimentation. This linkage and probable interaction may also bestow evolutionary advantage by minimising inter-individual variation in free thyroxine levels and by augmenting homeostasis.

Redefinition of Successful Treatment of Patients With Hypothyroidism. Is TSH the Best Biomarker of Euthyroidism?

In recent years evidence has accumulated supporting a revised view of the nature of euthyroidism and the biomarkers of thyroid function. Within the normal range, variations in thyroid hormone levels are associated with variations in clinical parameters and outcomes. There are therefore no readily identified individually specific optimum levels of thyroid hormones for any individual. Levels around the middle of the normal population range may best reflect euthyroidism. These levels may have evolutionary advantages on the basis that adverse outcomes often increase with divergence from such levels, and physiological processes tend to minimise such inter-individual and intra-individual divergence. In populations of predominantly untreated individuals, levels of thyroid hormones and in particular levels of free thyroxine (FT4) correlate more often with clinical parameters than do levels of thyrotropin (TSH). Levels of thyroid hormones may therefore be regarded as the best available biomarkers of euthyroidism and dysthyroidism. It follows that 'subclinical hypothyroidism' (normal FT4/raised TSH levels), rather than being an accurate marker of peripheral tissue hypothyroidism is more a marker of decreased thyroid reserve and prognosis. The recent evidence suggests that treatment of hypothyroxinemia, regardless of the TSH level, and monitoring therapy using FT4 and/or triiodothyronine levels, depending on the replacement regime, may result in more successful treatment of hypothyroidism than relying on thyrotropin levels for patient selection and subsequent treatment monitoring. The equivalents of mid-range levels of thyroid hormones (especially FT4), adjusted by individual comorbidity concerns, may be rational general replacement targets. These implications of the new evidence may create opportunities for novel trials of thyroid replacement therapy.

The Impact of Iodine Exposure in Excess on Hormonal Aspects and Hemato-Biochemical Profile in Rats

Excessive exposure of iodine over a time is well known to cause thyroid dysfunction, which may be followed by different effects on body organs. The present study aimed to illustrate the impacts of exposure of rats to excess iodine (above the tolerable range) and the reversibility of any negative impacts on hormonal profile related to thyroid besides cortisol and the hematological and biochemical parameters along with the histopathological alterations in the thyroid gland, liver, kidneys, and heart. Seventy-five rats were divided equally into three groups: Group 1 was control animals. Groups 2 and 3 received sodium iodide (NaI) orally at a dose of (35 and 70 mg/kg BW), which corresponded to (500 and 1000) times excess iodine from the physiological dose, respectively for 30 days, then the NaI administration stopped in the treated groups for 15 consecutive days. Blood and tissue samples were collected twice for various experimental tests after 30 and 15 days of exposure to excess iodine and stopping the exposure, respectively. Overall results revealed that excess iodine in both tested groups developed a hyperthyroid condition, hypercortisolism, relative polycythemia, neutropenia, elevation in serum liver and cardiac enzymes activities, hyperprotenemia, hyperglobulinemia, elevation in serum urea, and cardiac troponin I concentrations (p < 0.05). It was concluded that the excess iodine caused hyperthyroidism, which was associated with significant changes in erythrogram and leukogram and alterations in hepatic, renal, and cardiac functions in an iodine dose-dependent damage relationship and the most of negative impacts continued after stopping the administration.

The application of new concepts of the assessment of the thyroid state to pregnant women

Recently proposed concepts regarding the nature and assessment of the thyroid state have provided a model more consistent with empiric evidence. It now appears likely that there are no such entities as thyroid set points and individual euthyroidism. Rather than there being discrete thyroid states, peripheral organ parameters are associated with thyroid function in a continuous manner. Thyroid hormone levels and, in particular, levels of free thyroxine now appear to be superior to thyrotropin levels as indicators of the thyroid state. Complicating the assessment of the correlations of the thyroid state with pregnancy outcomes are the contribution of the placenta to maternal thyroid function, fetal thyroid development, the multiple potential pathways to any particular outcome, the likely presence of small critical periods of time, the differing genetics of fetal and maternal tissues, and the unreliability of thyroid hormone assays. Nevertheless, there is no apparent reason for there to be a change in pregnancy to the basic principles of thyroid hormone action. The relationships between mild abnormalities of the thyroid state and pregnancy outcomes and the value of treating such mild abnormalities remain uncertain and controversial. The evidence suggests that further investigation of these clinical questions might better be based on thyroid hormone, particularly free thyroxine, levels. In the investigation of borderline low thyroid states, the categories of subclinical hypothyroidism and isolated hypothyroxinemia might both be abandoned with attention being directed to low free thyroxine levels regardless of the thyroid-stimulating hormone (TSH) levels. For these changes to occur, there would ideally be improvements in the assays for free thyroxine in pregnancy. The evidence suggests that, just as in the non-pregnant situation, pregnancy guidelines based on thyrotropin levels may need revision.

Комплаєнс до лікування левотироксином хворих на гіпотиреоз під час пандемічної ери SARS-CoV-2

Актуальність. Гіпотиреоз потребує замісної терапії левотироксином (L-T4), щоб підтримувати в пацієнтів стан еутиреозу. Кількість госпіталізацій з приводу хронічних захворювань зменшилась протягом ери COVID-19. Мета дослідження— встановити частоту госпіталізацій пацієнтів з гіпотиреозом під час пандемії COVID-19 порівняно з аналогічним періодом часу в 2019 році. Матеріали та методи. Дослідження було проведене серед пацієнтів, які звернулись у відділення внутрішньої медицини університетської лікарні Abant Izzet Baysal та отримували замісну терапію L-T4 внаслідок гіпотиреозу. 108 пацієнтів з гіпотиреозом були включені в дослідження, підписавши інформаційну згоду. Пацієнти були розділені на дві групи відповідно до прихильності до лікування: хворі, які відповідали вимогам лікування, і хворі, які не дотримувались режиму лікування. Результати. Рівень тиреотропного гормона (ТТГ) перед включенням у дослідження у групі осіб, які відповідали вимогам лікування, був вірогідно нижчим, ніж у групі хворих, які не дотримувались режиму лікування (p<0,001). При цьому рівень вільного тироксину (вТ4) серед пацієнтів першої групи був вірогідно вищим, ніж в осіб другої групи (p=0,04). Медіана нерегулярного прийому левотироксину становила 35 (10–90) днів у групі осіб, які не відповідали вимогам лікування, і 0 (0–0) днів у групі осіб, які відповідали вимогам лікування (p<0,001). Кількість днів, коли левотироксин використовувався нерегулярно, вірогідно позитивно корелювала з останнім значенням ТТГ (r=0,564, p<0,001) та негативно— з останнім значенням вT4 (r=–0,492, p<0,001). Середній об’єм еритроцитів (MCV) у хворих першої та другої груп становив 85 (69,1–97,5) та 89 (66–96,6) фл відповідно (p=0,03). Ретроспективний характер та порівняно невелика кількість досліджуваних були двома основними обмеженнями проведеного дослідження. Висновки. Дотримання режиму лікування є дуже важливим для досягнення стану еутиреозу в пацієнтів з гіпотиреозом під час пандемії COVID-19. Підвищений рівень MCV може передбачати невідповідність лікування у хворих на гіпотиреоз.

Clinical Parameters Are More Likely to Be Associated with Thyroid Hormone Levels than with Thyrotropin Levels: A Systematic Review and Meta-Analysis

Background: Though the functional states of other endocrine systems are not defined on the basis of levels of controlling hormones, the assessment of thyroid function is based on levels of the controlling hormone thyrotropin (TSH). We, therefore, addressed the question as to whether levels of thyroid hormones [free thyroxine (fT4), total triiodothyronine (TT3)/free triiodothyronine (fT3)], or TSH levels, within and beyond the reference ranges, provide the better guide to the range of clinical parameters associated with thyroid status. Methods: A PubMed/MEDLINE search of studies up to October 2019, examining associations of levels of thyroid hormones and TSH, taken simultaneously in the same individuals, with clinical parameters was performed. We analyzed atrial fibrillation, other cardiac parameters, osteoporosis and fracture, cancer, dementia, frailty, mortality, features of the metabolic syndrome, and pregnancy outcomes. Studies were assessed for quality by using a modified Newcastle-Ottawa score. Preferred Reporting Items for Systematic Reviews and Meta-analyses guidelines were followed. A meta-analysis of the associations was performed to determine the relative likelihood of fT4, TT3/fT3, and TSH levels that are associated with the clinical parameters. Results: We identified 58 suitable articles and a total of 1880 associations. In general, clinical parameters were associated with thyroid hormone levels significantly more often than with TSH levels-the converse was not true for any of the clinical parameters. In the 1880 considered associations, fT4 levels were significantly associated with clinical parameters in 50% of analyses. The respective frequencies for TT3/fT3 and TSH levels were 53% and 23% (p < 0.0001 for both fT4 and TT3/fT3 vs. TSH). The fT4 and TT3/fT3 levels were comparably associated with clinical parameters (p = 0.71). More sophisticated statistical analyses, however, indicated that the associations with TT3/fT3 were not as robust as the associations with fT4. Conclusions: Thyroid hormones levels, and in particular fT4 levels, seem to have stronger associations with clinical parameters than do TSH levels. Associations of clinical parameters with TSH levels can be explained by the strong negative population correlation between thyroid hormones and TSH. Clinical and research components of thyroidology currently based on the measurement of the thyroid state by reference to TSH levels warrant reconsideration.

Population data provide evidence against the presence of a set point for hemoglobin levels or tissue oxygen delivery

Hemoglobin levels are believed to be regulated as per a set point model of regulation. This model of regulation, by which specific levels of a parameter are targeted and defended by physiological systems, implies a particular population correlation between the parameter and its controlling hormone. Empirical population correlations of other parameters and their controlling hormones, have denied the presence of such set point-based regulation. To assess if hemoglobin is regulated according to a set point model we performed a systematic search of PubMed/MEDLINE and Web of Science identifying relevant reports published up to November 2018. Population hemoglobin/erythropoietin level correlations were retrieved, and these empirically derived correlations were compared with the positive correlation implied by a set point model of regulation. Authors of papers containing potentially suitable data were contacted with requests for further analyses, and a meta-analysis was performed. Twelve correlations between hemoglobin and erythropoietin levels from eleven papers were analyzed. None of these correlations were significantly positive, three, restricted to the normal range of hemoglobin, were significantly negative. All but one of the other correlations showed a negative trend. New analyses of previously published data sets resulted in similar findings. In particular a new analysis of large data sets of males (n = 2417) and females (n = 2592) with normal range hemoglobin levels, revealed significantly negative correlations. A meta-analysis of our results indicated that the data overall are not consistent with a positive relationship between hemoglobin and erythropoietin (P < 0.0001). Population data indicate that individuals do not have set point levels of hemoglobin.