Seasonal variations in TSH serum levels in athyreotic patients under L-thyroxine replacement monotherapy

A Systematic Review and Meta-Analysis of Free Triiodothyronine (FT3) Levels in Humans Depending on Seasonal Air Temperature Changes: Is the Variation in FT3 Levels Related to Nonshivering Thermogenesis?

Thyroid hormones play a crucial role in regulating normal development, growth, and metabolic function. However, the controversy surrounding seasonal changes in free triiodothyronine (FT3) levels remains unresolved. Therefore, the aim of this study was to conduct a systematic review and meta-analysis of variations in FT3 levels in relation to seasonal air temperatures in the context of current knowledge about its role in nonshivering thermogenesis. Ten eligible articles with a total of 336,755 participants were included in the meta-analysis. The studies were categorized into two groups based on the air temperature: "Cold winter", where the winter temperature fell below 0 °C, and "Warm winter", where the winter temperature was above 0 °C. The analysis revealed that in cold regions, FT3 levels decreased in winter compared to summer (I2 = 57%, p < 0.001), whereas in warm regions, FT3 levels increased during winter (I2 = 28%, p < 0.001). These findings suggest that seasonal variations in FT3 levels are likely to be influenced by the winter temperature. Considering the important role of the FT3 in the nonshivering thermogenesis process, we assume that this observed pattern is probably related to the differences in use of thyroid hormones in the brown adipose tissue during adaptive thermogenesis, which may depend on intensity of cold exposure.

Seasonal variations in levels of human thyroid-stimulating hormone and thyroid hormones: a meta-analysis

Seasonal dynamics in biological functions of mammals is regulated by melatonin-mediated circannual fluctuations in the secretion of thyroid-stimulating hormone (TSH) and thyroid hormones. Most anatomical and molecular structures responsive to photoperiod and melatonin secretion changes and the associated receptors are preserved in modern humans. This work aimed to determine the seasonal dynamics of TSH and thyroid hormone levels (total triiodothyronine (T3), free triiodothyronine (FT3), thyroxine (T4), free thyroxine (FT4) and to investigate the dependence of these variations on gender, age and amplitude of meteorological fluctuations. A meta-analysis of 13 panel and 7 cross-sectional studies was performed using Review Manager 5.3 (Cochrane Library). We found that circulating TSH levels were higher in winter than in other seasons, and FT4 levels were higher in autumn than in winter. T4 level had no pronounced seasonal dynamics. The level of circulating T3 was significantly higher in winter than in summer and FT3 levels were lower in summer than in autumn and spring. In addition, analysis of TSH seasonal dynamics (winter vs summer) accounting for gender differences showed pronounced increases in TSH levels during winter in women, but not in men; and also significant increases in FT4 levels during summer in men, but not in women. Seasonal dynamics of FT3 and T4 did not depend on gender. Seasonal dynamics of TSH did not change with respect to age. We also found that the extent of the seasonal dynamics of TSH is influenced by the extent of the annual dynamics of the partial density of oxygen in the air, as well as the magnitude of the annual dynamic of meteorological factors that determine it (atmospheric pressure and relative humidity).

Within-Person Variation in Serum Thyrotropin Concentrations: Main Sources, Potential Underlying Biological Mechanisms, and Clinical Implications

BACKGROUND

Individuals exhibit fluctuations in the concentration of serum thyroid-stimulating hormone (TSH) over time. The scale of these variations ranges from minutes to hours, and from months to years. The main factors contributing to the observed within-person fluctuations in serum TSH comprise pulsatile secretion, circadian rhythm, seasonality, and ageing. In clinical practice and clinical research however, such within-person biological variation in serum TSH concentrations is often not considered. The aim of this review is to present an overview of the main sources of within-person variation in TSH levels, as well as the potential underlying biological mechanisms, and the clinical implications.

SUMMARY

In euthyroid individuals, the circadian rhythm, with a nocturnal surge around 02:00-04:00 h and a nadir during daytime has the greatest impact on variations in serum TSH concentrations. Another source of within-person variation in TSH levels is seasonality, with generally higher levels during the cold winter months. Since TSH is secreted in a pulsatile manner, TSH levels also fluctuate over minutes. Furthermore, elevated TSH levels have been observed with ageing. Other factors that affect TSH levels include thyroid peroxidase (TPO)-antibody positivity, BMI, obesity, smoking, critical illness, and many xenobiotics, including environmental pollutants and drugs. Potential underlying biological mechanisms of within-person variation in TSH levels can be safely concluded from the ability of TSH to respond quickly to changes in cues from the internal or external environment in order to maintain homeostasis. Such cues include the biological clock, environmental temperature, and length of day. The observed increase in TSH level with ageing can be explained at a population level and at an organism level. In clinical practice, the season for thyroid testing can influence a patient's test result and it occurs frequently that subclinical hypothyroid patients normalize to euthyroid levels over time without intervention.

CONCLUSIONS

Serum TSH concentrations vary over time within an individual, which is caused by multiple different internal and external factors. It is important to take the within-person variations in serum TSH concentrations into account when testing a patient in clinical practice, but also in performing clinical research.

Prevalence of gastrointestinal disorders having an impact on tablet levothyroxine absorption: should this formulation still be considered as the first-line therapy?

PURPOSE

In patients with hypothyroidism, levothyroxine (LT4) is the treatment of choice, and tablets are the most commonly prescribed formulation. Despite multiple scenarios being reported in the literature with impaired tablet absorption and likely missed TSH targets, it is yet unclear what the implications are for clinical practice and the role of liquid solution (LS) and soft gel (SG) formulations. We have thus conducted a narrative review evaluating the prevalence within the general population of gastrointestinal disorders impacting tablet LT4 absorption.

METHODS

PubMed and Google Scholar were searched until December 2019 for systematic reviews and meta-analyses on the topic. If they could not be retrieved, other types of manuscripts were searched.

RESULTS

Lactose malabsorption and Helicobacter pylori infection represented the most common disorders, with a global prevalence of 68% and 48%, respectively. The prevalence of other conditions, including autoimmune gastritis, bariatric surgery, celiac disease, gastroparesis, giardiasis, liver cirrhosis, or ulcerative colitis, was lower than 20%. Data at regional and country levels were found to be heterogeneous, but at least one in five patients was diagnosed with one disorder.

CONCLUSIONS

The worldwide prevalence of gastrointestinal disorders associated with tablet LT4 malabsorption, including lactose malabsorption and Helicobacter pylori infection, is high. Interactions with drugs or food can further increase this risk. Considering that all studies investigating the impact of switching patients from tablet to LS or SG found an improved thyroid balance, the latter formulations should be considered as first-line therapy for managing hypothyroidism.

Changes in TSH levels in athyreotic patients with differentiated thyroid cancer during levothyroxine therapy: influence on dose adjustments

PURPOSE

The aim of the study was to describe the spontaneous TSH level variations and levothyroxine dose adjustments in athyreotic patients with differentiated thyroid cancer (DTC) in real-life practice.

METHODS

Patients with DTC were retrospectively evaluated at a tertiary referral center between October 2006 and November 2013. Hormone measurements (TSH and FT4 serum levels), L-T4 prescription information (dose per kg per day) and other medications were recorded at 1 month and 3, 12, 24, 36 and 48 months after primary treatment (surgery ± radioiodine therapy).

RESULTS

The cohort was composed of 452 patients; about 20% of patients with stable levothyroxine dose have clinically meaningful spontaneous TSH variations (defined as ΔTSH > 2 mcUI/mL) at yearly follow-up visit. Furthermore, about 25% of athyreotic DTC patients with stable dose have a ΔTSH > 1.5 mcUI/mL and about 40% a ΔTSH > 1 mcUI/mL during each follow-up visit. We further investigated whether this TSH variation would lead to subsequent dose changes. About 19.9-37.7% of DTC patients on stable LT4 dose on the previous visit had their levothyroxine dose reduced, while 7.8-14.9% increased due to TSH variations. We further evaluated the decision to change the dose in relation with the age-specific TSH range. Up to 77.2% of patients had their dose adjusted due to TSH falling below the age-specific range.

CONCLUSIONS

Spontaneous serum TSH variations determine levothyroxine replacement therapy in athyreotic patients with DTC, requiring multiple dose changes.

Interconnection between circadian clocks and thyroid function

Circadian rhythmicity is an approximately 24-h cell-autonomous period driven by transcription-translation feedback loops of specific genes, which are referred to as 'circadian clock genes'. In mammals, the central circadian pacemaker, which is located in the hypothalamic suprachiasmatic nucleus, controls peripheral circadian clocks. The circadian system regulates virtually all physiological processes, which are further modulated by changes in the external environment, such as light exposure and the timing of food intake. Chronic circadian disruption caused by shift work, travel across time zones or irregular sleep-wake cycles has long-term consequences for our health and is an important lifestyle factor that contributes to the risk of obesity, type 2 diabetes mellitus and cancer. Although the hypothalamic-pituitary-thyroid axis is under the control of the circadian clock via the suprachiasmatic nucleus pacemaker, daily TSH secretion profiles are disrupted in some patients with hypothyroidism and hyperthyroidism. Disruption of circadian rhythms has been recognized as a perturbation of the endocrine system and of cell cycle progression. Expression profiles of circadian clock genes are abnormal in well-differentiated thyroid cancer but not in the benign nodules or a healthy thyroid. Therefore, the characterization of the thyroid clock machinery might improve the preoperative diagnosis of thyroid cancer.

Semi-annual seasonal pattern of serum thyrotropin in adults

Circannual rhythmicity in thyroid-stimulating hormone (TSH) secretion is proposed, whereas evidences on seasonal peripheral thyroid hormones’ fluctuation are contradictory. This study was designed to evaluate hypothalamic-pituitary-thyroid (HPT) seasonal secretion pattern using a big data approach. An observational, retrospective, big data trial was carried out, including all TSH measurements performed in a single laboratory between January 2010 and December 2017. A large dataset was created matching TSH data with patients’ age, gender, environmental temperature exposure, and free triiodothyronine (fT3) and free thyroxine (fT4) when available. The trend and seasonal distributions were analysed using autoregressive integrated moving average models. A total of 1,506,495 data were included in the final database with patients mean age of 59.00 ± 18.44 years. The mean TSH serum levels were 2.08 ± 1.57 microIU/mL, showing a seasonal distribution with higher levels in summer and winter seasons, independently from age, gender and environmental temperatures. Neither fT3 nor fT4 showed a seasonal trend. TSH seasonal changes occurred independently from peripheral thyroid hormone variations, gender, age and environmental temperatures. Although seasonal TSH fluctuation could represent a residual ancestral mechanism to maintain HPT homeostasis, the underlying physiological mechanism remains unclear and specific studies are needed to clarify its impacting role in humans.

Is There Seasonality in Hypothyroidism? A Google Trends Pilot Study

INTRODUCTION

There may be seasonality in thyroid diseases and internet search data may provide information on disease patterns. In this study we used data from internet searches on hypothyroidism to assess seasonality in this disease.

METHODS

We collected worldwide data, as well as data for countries in the southern hemisphere (Brazil, South Africa, and Australia), covering 15 years, from Google Trends with the search term "hypothyroidism+thyroiditis (the commonest cause of hypothyroidism)" and "fatigue+weakness (the commonest symptoms of hypothyroidism)". We looked for periodicity in relevant internet searches by calculating autocorrelations; we also looked at the cross-correlation of internet searches for "hypothyroidism+thyroiditis" and "fatigue+weakness" and we compared the results by season with the Kruskall-Wallis test.

RESULTS

There was periodicity in the relevant internet searches and strong cross-correlations between internet searches for "hypothyroidism+thyroiditis" and "fatigue+weakness" worldwide and for South Africa and Australia. In both the northern and the southern hemispheres there were significantly more hypothyroidism-related internet searches during spring (p<0.05).

CONCLUSION

Hypothyroidism was more popular in internet searches at springtime in the northern and the southern hemispheres. Thus, although this analysis is coarse, it seems that some seasonality can be inferred on hypothyroidism, taking into account the limitations of our approach.

Relation of Different Components of Climate with Human Pituitary-Thyroid Axis and FT3/FT4 Ratio: A Study on Euthyroid and SCH Subjects in Two Different Seasons

BACKGROUND

Various changes in thyroid hormones (TH) and thyroid-stimulating hormone (TSH) level were observed in different seasons among euthyroid and hypothyroid subjects living in areas with an extreme temperature difference between summer and winter.

OBJECTIVES

This study aims at finding the effect of temperate climate on the seasonal variations of TSH and TH in euthyroid and subclinical hypothyroidism (SCH) subjects and at evaluating if the test season has an effect on the number of subjects diagnosed as SCH. It basically focuses on the relation of different components of climate with TH and TSH.

METHOD

In a prospective study on 152 healthy (euthyroid) volunteers and 25 SCH subjects, the serum hormone levels (TSH, FT4, and FT3) were measured in both the summer and winter seasons and correlated with all the climate components using Pearson's correlation coefficient. The effect of duration of outdoor exposure on hormone levels was compared using a paired sample t-test (P < 0.05).

RESULTS

Small but statistically significant increased FT3 level and decreased FT4 level were observed during the winter season in euthyroid and SCH subjects, respectively. There was a significant negative correlation between FT3 and FT3/FT4 ratio with temperature and sunshine duration and a positive correlation with humidity and atmospheric pressure. A positive correlation was found between FT4 and sunshine duration.

CONCLUSION

The climate components contributed to the slight variance in hormone levels in different seasons, and the effect was mostly on peripheral conversion of FT4 to FT3 rather than the pituitary-thyroid axis leading to slightly higher FT3 in winter. Seasonal variation does not affect the diagnosis of SCH cases.

Seasonal Changes in Serum Thyrotropin Concentrations Observed from Big Data Obtained During Six Consecutive Years from 2010 to 2015 at a Single Hospital in Japan

BACKGROUND

This study analyzed big data for serum thyrotropin (TSH), free triiodothyronine (fT3), and free thyroxine (fT4) concentrations in patients who had attended the outpatient clinic of Ito Hospital (Tokyo, Japan) during a recent six-year period (between January 1, 2010, and December 31, 2015) in order to investigate for seasonal changes.

METHODS

The serum TSH concentrations were reviewed for all 135,417 patients aged >20 years. Patients with any thyroid diseases were included, irrespective of whether they were receiving drug therapy. In total 1,637,721 serum samples were analyzed for TSH, 1,626,269 for fT3, and 1,669,381 for fT4.

RESULTS

It was observed that the TSH concentrations showed annual changes during the six-year period. They decreased during the summer, while they increased during the winter. The TSH concentrations were negatively correlated with the daily temperatures (Spearman rank correlation coefficient -0.4486; p < 0.0001). The same applied for the correlation between fT3 concentrations and daily temperatures.

CONCLUSIONS

The fact that the TSH concentrations show annual changes in areas where the temperature ranges during the year are rather wide should be borne in mind for interpretation of results.

SEASONAL VARIATION OF VITAMIN D AND SERUM THYROTROPIN LEVELS AND ITS RELATIONSHIP IN A EUTHYROID CAUCASIAN POPULATION

OBJECTIVE

It is unclear whether seasonal variations in vitamin D concentrations affect the hypothalamo-pituitary-thyroid axis. We investigated the seasonal variability of vitamin D and serum thyrotropin (TSH) levels and their interrelationship.

METHODS

Analysis of 401 patients referred with nonspecific symptoms of tiredness who had simultaneous measurements of 25-hydroxyvitamin D3 (25[OH]D3) and thyroid function. Patients were categorized according to the season of blood sampling and their vitamin D status.

RESULTS

25(OH)D3 levels were higher in spring-summer season compared to autumn-winter (47.9 ± 22.2 nmol/L vs. 42.8 ± 21.8 nmol/L; P = .02). Higher median (interquartile range) TSH levels were found in autumn-winter (1.9 [1.2] mU/L vs. 1.8 [1.1] mU/L; P = .10). Across different seasons, 25(OH)D3 levels were observed to be higher in lower quartiles of TSH, and the inverse relationship was maintained uniformly in the higher quartiles of TSH. An independent inverse relationship could be established between 25(OH)D3 levels and TSH by regression analysis across both season groups (autumn-winter: r = -0.0248; P<.00001 and spring-summer: r = -0.0209; P<.00001). We also observed that TSH varied according to 25(OH)D3 status, with higher TSH found in patients with vitamin D insufficiency or deficiency in comparison to patients who had sufficient or optimal levels across different seasons.

CONCLUSION

Our study shows seasonal variability in 25(OH)D3 production and TSH secretion in euthyroid subjects and that an inverse relationship exists between them. Further studies are needed to see if vitamin D replacement would be beneficial in patients with borderline thyroid function abnormalities.

ABBREVIATIONS

25(OH)D2 = 25-hydroxyvitamin D2; 25(OH)D3 = 25-hydroxyvitamin D3; AITD = autoimmune thyroid disease; FT4 = free thyroxine; TFT = thyroid function test; TSH = thyrotropin; UVB = ultraviolet B.

Thyroid Allostasis-Adaptive Responses of Thyrotropic Feedback Control to Conditions of Strain, Stress, and Developmental Programming

The hypothalamus-pituitary-thyroid feedback control is a dynamic, adaptive system. In situations of illness and deprivation of energy representing type 1 allostasis, the stress response operates to alter both its set point and peripheral transfer parameters. In contrast, type 2 allostatic load, typically effective in psychosocial stress, pregnancy, metabolic syndrome, and adaptation to cold, produces a nearly opposite phenotype of predictive plasticity. The non-thyroidal illness syndrome (NTIS) or thyroid allostasis in critical illness, tumors, uremia, and starvation (TACITUS), commonly observed in hospitalized patients, displays a historically well-studied pattern of allostatic thyroid response. This is characterized by decreased total and free thyroid hormone concentrations and varying levels of thyroid-stimulating hormone (TSH) ranging from decreased (in severe cases) to normal or even elevated (mainly in the recovery phase) TSH concentrations. An acute versus chronic stage (wasting syndrome) of TACITUS can be discerned. The two types differ in molecular mechanisms and prognosis. The acute adaptation of thyroid hormone metabolism to critical illness may prove beneficial to the organism, whereas the far more complex molecular alterations associated with chronic illness frequently lead to allostatic overload. The latter is associated with poor outcome, independently of the underlying disease. Adaptive responses of thyroid homeostasis extend to alterations in thyroid hormone concentrations during fetal life, periods of weight gain or loss, thermoregulation, physical exercise, and psychiatric diseases. The various forms of thyroid allostasis pose serious problems in differential diagnosis of thyroid disease. This review article provides an overview of physiological mechanisms as well as major diagnostic and therapeutic implications of thyroid allostasis under a variety of developmental and straining conditions.