Effects of Metformin on TSH Levels and Benign Nodular Goiter Volume in Patients Without Insulin Resistance or Iodine Insufficiency

The impact of metformin on weight and metabolic parameters in patients with obesity: A systematic review and meta-analysis of randomized controlled trials

There are conflicting data on the weight-reducing potential of metformin (MTF) in nondiabetic patients with obesity. The purpose of this systematic review and meta-analysis was to evaluate the effect of MTF on weight and cardiometabolic parameters in adults with overweight/obesity with or without nonalcoholic fatty liver disease (NAFLD) (CRD42018085512). We included randomized controlled trials (RCTs) in adults without diabetes mellitus, with mean body mass index (BMI) ≥ 25 kg/m2, with or without NAFLD, comparing MTF to placebo/control, lifestyle modification (LSM) or a US Food and Drug Administration-approved anti-obesity drug, reporting on weight or metabolic parameters, and extending over at least 3 months. We conducted a systematic search in MEDLINE, EMBASE, PubMed and the Cochrane Library without time limitation (until March 2022). We screened and selected eligible articles, abstracted relevant data, and assessed the risk of bias. All steps were in duplicate and independently. We conducted a random-effects model meta-analysis using Review Manager version 5.3, with prespecified subgroup analyses in case of heterogeneity. We identified 2650 citations and included 49 trials (55 publications). Compared to placebo, MTF was associated with a significant reduction in BMI (mean difference [MD] -0.56 [-0.74, -0.37] kg/m2; p < 0.0001), at doses ranging from 500 to 2550 mg/day, and with a significant percentage change in BMI of -2.53% (-2.90, -2.17) at the dose 1700 mg/day. There was no interaction by baseline BMI, MTF dose or duration, nor presence or absence of NAFLD. There was no significant difference between MTF and LSM. Orlistat was more effective than MTF (at doses of 1000-1700 mg/day) in terms of weight loss, with an MD in BMI of -3.17 (-5.88; -0.47) kg/m2, favouring the former. Compared to placebo/control, MTF improved insulin parameters, while no effect was detected when compared to LSM. A few small trials showed heterogenous effects on liver parameters in patients with NAFLD treated with MTF compared to placebo/control. There was a large variability in the expression of outcome measures and RCTs were of low quality. In conclusion, MTF was associated with a modest weight reduction in obese nondiabetic patients. Further high-quality and better powered studies are needed to examine the impact of MTF in patients with insulin resistance and NAFLD.

Risk factors associated with the prevalence of thyroid nodules in adults in Northeast China: a cross-sectional population-based study

OBJECTIVES

This study examined the association between anthropometric measurements, lifestyle factors and the prevalence of thyroid nodules among adults in Northeast China.

DESIGN

We employed a cross-sectional approach involving a questionnaire survey, which focused on participants' living habits, and a physical examination that included anthropometry and ultrasound imaging.

SETTING

The data were procured during multiple trips by medical teams from the first hospital of China Medical University to towns in Northeast China.

PARTICIPANTS

Of the 1092 participants, 489 did not have thyroid nodules (mean age: 54.02±11.49 years; 297 females (60.7%)), 99 had single thyroid nodules (mean age: 58.19±10.77 years; 59 females (59.6%)) and 504 had multiple thyroid nodules (mean age: 60.05±10.68 years; 394 females (78.2%)). Inclusion criteria mandated participants be over 20 years old without other medical conditions. We excluded individuals who had undergone surgical resection for thyroid nodules.

RESULTS

The prevalence of thyroid nodules was significantly associated with being female (OR 2.569, 95% CI 1.937 to 3.405, p<0.001) and increased age (OR 1.054, 95% CI 1.041 to 1.066, p<0.001). This association was more pronounced in those with multiple thyroid nodules. For males under 60, non-smoking was inversely correlated with the prevalence of multiple thyroid nodules (OR 0.321, 95%CI 0.149 to 0.69, p<0.05). For females under 60, diastolic blood pressure (DBP) was significantly linked with the prevalence of thyroid nodules (OR 0.978, 95% CI 2.614 to 2.705, p<0.05).

CONCLUSIONS

Besides gender and age, the prevalence of thyroid nodules in Northeast China correlates with smoking habits and DBP.

Impact of Bariatric Surgery on Thyroid Morphology and Thyroid Function

INTRODUCTION

Few studies have evaluated the impact of bariatric surgery (BS) on thyroid function and morphology, and how it correlates to inflammatory and metabolic markers. We aimed to evaluate all those parameters together.

METHODS

A longitudinal study included 70 patients with severe obesity. The bariatric group (BG) enrolled 40 patients who underwent BS, and the control group (CG) enrolled 30 patients who did not undergo BS. Both were submitted (pre- and 2nd-year) to thyroid ultrasound and laboratory analyses to determine the levels of thyroid hormones, inflammatory, and metabolic markers.

RESULTS

Thyroid volume (TV) decreased after BS (-1.5 cm3), differing significantly from the CG (+0.6 cm3; p = 0.003). ΔTV was independently and positively correlated with ΔHOMA-IR (0.41 (0.11/7.16) p = 0.007) and ΔIL6 (0.02 (0.01/0.3) p = 0.016). A nonsignificant correlation between ΔTV and ΔBMI was detected (0.38 (-0.01/0.09) p = 0.152). We also observed a negative correlation between ΔTV and ΔTSH (-2.03 (-2.87/-1.19) p = 0.000) and ΔT3/T4 ratio (-0.06 (-0.09/-0.02) p = 0.001). TSH had a nonsignificant reduction with BS (-0.3872 vs. -0.2483 p = 0.128). The conversion of T4 to T3 had a significant increase after BS, as demonstrated by the T3/T4 ratio (+5.16 p = 0.01). Despite an increase in the prevalence of thyroid nodules in the BG, it was not statistically significant (p = 0.340).

CONCLUSION

BS was associated with a reduction in TV and a nonstatistically significant reduction in TSH. The variations in TV were related to the metabolic markers and inflammatory changes. An increase in the conversion of T4 to T3 with BS was detected, possibly related to inflammatory improvement.

Effects of Slow-Acting Metformin Treatment on the Hormonal and Morphological Thyroid Profile in Patients with Insulin Resistance

Metformin appears to reduce TSH levels in untreated hypothyroid patients. In contrast, in euthyroid patients with type 2 diabetes mellitus (T2DM), metformin is initially devoid of effects on TSH. However, it is followed by a significant reduction in TSH level after twelve months of treatment. Additionally, some evidence suggests that metformin may also improve thyroid morphological abnormalities. This study aimed to evaluate the effects of metformin not only on TSH and thyroid hormone values, but also on thyroid volume and nodules. A total of 50 patients (mean age: 36.9 ± 12.8 years) with insulin resistance (homeostatic model assessment (HOMA) index ≥2.5) and with thyroid uninodular disease were recruited for this study. They were prescribed slow-acting metformin at a daily dose of 500 mg for six months. Treatment with metformin in euthyroid patients with uninodular thyroid disease and insulin resistance reduces TSH levels, increases FT4 and FT3 values, and decreases thyroid and nodule volumes. These data suggest that metformin may be an effective drug not only for the treatment of T2DM and metabolic syndrome, but also for thyroid disease.

A breakthrough-like effect of metformin reduces peripheral resistance to triiodothyronine in euthyroid, non-insulin-resistant, type 2 diabetic patients

BACKGROUND

Type 2 diabetes is characterized, beyond the insulin resistance, by polyhormonal resistance. Thyroid hormonal resistance has not yet been described in this population of patients. Metformin is used to decrease insulin resistance, and at present, it is assumed to influence the effect of triiodothyronine, as well.

METHODS

In this open-label, pilot, hypothesis-generating, follow-up study, 21 patients were included; all of them were euthyroid with drug naïve, newly diagnosed type 2 diabetes. Before and after 4 weeks of metformin therapy, fructosamine, homeostasis model assessment for insulin resistance (HOMA-IR), thyroid hormones, T3/T4 ratio, and TSH, as well as blood pressure and heart rate using ambulatory blood pressure monitor were measured. We also conducted an in vitro study to investigate the possible mechanisms of T3 resistance, assessing T3-induced Akt phosphorylation among normal (5 mM) and high (25 mM) glucose levels with or without metformin treatment in a human embryonal kidney cell line.

RESULTS

Metformin decreased the level of T3 (P < 0.001), the ratio of T3/T4 (P = 0.038), fructosamine (P = 0.008) and HOMA-IR (P = 0.022). All these changes were accompanied by an unchanged TSH, T4, triglyceride, plasma glucose, bodyweight, blood pressure, and heart rate. In our in vitro study, T3-induced Akt phosphorylation decreased in cells grown in 25 mM glucose medium compared to those in 5 mM. Metformin could not reverse this effect.

CONCLUSION

Metformin seems to improve T3 sensitivity in the cardiovascular system in euthyroid, type 2 diabetic patients, the mechanism of which may be supracellular.

TSH lowering effects of metformin: a possible mechanism of action

Preliminary clinical evidence suggests that metformin has TSH lowering effects in patients with T2DM and hypothyroidism or in those with TSH serum levels in the upper normal value. Also, metformin may exert a protective role against thyroid nodules growth in patients without insulin-resistance. The cross-talk between tyrosine kinase receptors and the G protein-coupled receptors (which the TSHR belongs to) has been already shown and IRS1 may represent the hub link between TSHR and IR pathways. By influencing IRS1 phosphorylation pattern, metformin may sensitize TSHR to TSH, thus explaining the findings of clinical studies. However, the existence of this molecular pathway must be confirmed through proper studies and further prospective randomized placebo-controlled studies are needed to confirm this hypothesis.

Exploring the Effect of Metformin to Lower Thyroid-Stimulating Hormone in Euthyroid and Hypothyroid Type-2 Diabetic Patients

Introduction

Metformin is one of the safest, first-line oral hypoglycemic agents used in type-2 diabetes mellitus patients. This study aims to study the effect of metformin on thyroid-stimulating hormone (TSH) in hypothyroid and euthyroid individuals, as both these diseases have an increased prevalence and coexistence.

Method

This hospital-based study was conducted in Jinnah Allama Iqbal Institute of Diabetes and Endocrinology (JAIDE), Allama Iqbal Medical College/Jinnah Hospital Lahore, Pakistan, from October 2019 to April 2020. One hundred and sixty type-2 diabetic participants, aged 25-60 years and meeting the inclusion criteria were enrolled in the study after informed consent. They were divided into two groups, the hypothyroid group who were already on levothyroxine therapy and had a stable TSH in the normal range, and a euthyroid group who had no thyroid dysfunction. Both the groups were started on metformin therapy for the control of type-2 diabetes mellitus and followed for six months. Their blood samples for TSH and free thyroid hormone (fT4) were drawn both prior to and after the study period.

Results

Out of the 160 type-2 diabetic patients, TSH levels showed a significant reduction in the hypothyroid patients (2.33 ± 0.70, p < 0.001) with no significant changes in the euthyroid patients (3.87 ± 0.40, p = 0.206) following six months of metformin therapy. However, there was no significant difference in the fT4 levels in either of the groups.

Conclusion

Metformin has the effect of significantly lowering TSH levels in hypothyroid individuals. However, no such effect was observed in euthyroid patients.

The role of thyroid hormone in metabolism and metabolic syndrome

Metabolic syndrome (MetS) and thyroid dysfunction are common in clinical practice. The objectives of this review are to discuss some proposed mechanisms by which thyroid dysfunctions may lead to MetS, to describe the bidirectional relationship between thyroid hormones (THs) and adiposity and finally, to resume a list of recent studies in humans that evaluated possible associations between thyroid hormone status and MetS or its clinical components. Not solely THs, but also its metabolites regulate metabolic rate, influencing adiposity. The mechanisms enrolled are related to its direct effect on adenosine triphosphate (ATP) utilization, uncoupling synthesis of ATP, mitochondrial biogenesis, and its inotropic and chronotropic effects. THs also act controlling core body temperature, appetite, and sympathetic activity. In a bidirectional way, thyroid function is affected by adiposity. Leptin is one of the hallmarks, but the pro-inflammatory cytokines and also insulin resistance impact thyroid function and perhaps its structure. MetS development and weight gain have been positively associated with thyroid-stimulating hormone (TSH) in several studies. Adverse glucose metabolism may be related to hyperthyroidism, but also to reduction of thyroid function or higher serum TSH, as do abnormal serum triglyceride levels. Hypo- and hyperthyroidism have been related to higher blood pressure (BP), that may be consequence of genomic or nongenomic action of THs on the vasculature and in the heart. In summary, the interaction between THs and components of MetS is complex and not fully understood. More longitudinal studies controlling each of all confounding variables that interact with endpoints or exposure factors are still necessary.