Thyroid function and risk of all-cause and cardiovascular mortality: a prospective population-based cohort study

The Effect of Thyroid Lobe Volume on the Common Carotid Artery Blood Flow in Thyroidectomy Position

Background/Objectives: This study investigates the effect of thyroid lobe size on common carotid artery hemodynamics during thyroidectomy. While prior research has reported reduced carotid blood flow during the procedure, the impact of thyroid size remains unclear. We hypothesized that larger thyroid lobes may influence carotid flow dynamics via external compression. Methods: Adult patients undergoing elective thyroidectomy were prospectively included. Doppler ultrasonography measured carotid artery diameters and flow characteristics at three time points: before anesthesia induction, after induction, and after surgical positioning. Regional cerebral oximetry was recorded. Each carotid artery was analyzed separately. Results: Data from 202 carotid arteries (132 patients) were analyzed. Baseline carotid diameters and flow velocities were similar between patients with normal and large thyroid lobes. Anesthesia induction reduced flow velocities in all patients. After surgical positioning, patients with large thyroid lobes had significantly increased peak systolic velocity, leading to an overestimation of carotid blood flow, when using formula-based calculations. Manually traced Velocity Time Integral confirmed the increase in peak systolic velocity and a shortened systolic/diastolic ratio in these patients. Receiver operating characteristic analysis identified a thyroid lobe volume cutoff of 19.7 mL (AUC: 0.93, Sensitivity: 85%, Specificity: 98%). Regional cerebral oxygen saturation remained unchanged (p > 0.05). Conclusions: Larger thyroid lobes are associated with altered carotid flow dynamics during thyroidectomy, emphasizing diastolic flow. While these findings provide insight into thyroid-related hemodynamic changes, their applicability to patients with pre-existing carotid stenosis or peripheral artery disease remains uncertain, as our study population did not include such cases.

Associations between peripheral thyroid sensitivity and all-cause and cardiovascular mortality in the US adults with metabolic syndrome

Background

The relationship between peripheral sensitivity to thyroid hormones, as indicated by the ratio of free triiodothyronine (fT3) to free thyroxine (fT4) (fT3/fT4), and the prognosis of metabolic syndrome (MetS) remains unclear.

Methods

This study utilized data from the National Health and Nutrition Examination Survey (NHANES) conducted between 2007 and 2012. MetS was defined based on the criteria established by the National Cholesterol Education Program Adult Treatment Panel III (NCEP-ATP III). Kaplan-Meier survival curves, restricted cubic spline (RCS) analysis, and Cox proportional hazards models were employed to investigate the association between peripheral thyroid sensitivity and mortality outcomes among adults with MetS.

Results

A total of 3,101 adult participants (1,594 males and 1,507 females; median age: 52.00 years) with MetS were included in the analysis. Multivariate Cox regression analysis revealed that elevated levels of fT4 were positively associated with increased risks of both all-cause and cardiovascular mortality in the MetS population [adjustedhazard ratio (aHR): 2.74, 95% confidence interval (CI): 1.94-3.87, p < 0.001 for all-cause mortality; aHR: 3.93, 95% CI: 2.07-7.45, p < 0.001 for cardiovascular mortality]. Conversely, higher levels of fT3 and the fT3/fT4 ratio were found to be protective factors, reducing the mortality risk in the MetS population (fT3: aHR: 0.76, 95% CI: 0.57-0.99, p = 0.046 for all-cause mortality; fT3/fT4 ratio: aHR: 0.75, 95% CI: 0.67-0.85, p < 0.001 for all-cause mortality; aHR: 0.66, 95% CI: 0.52-0.83, p < 0.001 for cardiovascular mortality). The fT3/fT4 ratio exhibited a nonlinear association with all-cause mortality, but a linear and inverse association with cardiovascular mortality.

Conclusion

The findings of this study suggest that higher peripheral thyroid sensitivity, as indicated by the fT3/fT4 ratio, may be associated with reduced mortality risks among adults with MetS. Further research is warranted to validate these associations.

Relationship of Thyroid Volume and Function with Ankle-Brachial Index, Toe-Brachial Index, and Toe Pressure in Euthyroid People Aged 18-65

Background and Objectives: The interrelationship between thyroid function and the state of the cardiovascular system has been investigated both in preclinical and human studies. However, it remains unclear whether there is any association between thyroid hormones and features of subclinical cardiovascular dysfunction in euthyroid patients. Material and Methods: This study involved 45 people (females: 57.8%) with no thyroid disease who, during planned hospitalization, underwent thyroid ultrasound, determination of biochemical parameters of thyroid function, and measurement of ankle-brachial index (ABI) and toe-brachial index (TBI). People with signs of acute illness or a deterioration of their health were excluded. Results: Significant correlations were found between free triiodothyronine (FT3) and several parameters of both ABI (R = 0.347; p = 0.019 for the mean ABI taken from right side and left side values) and TBI (R = 0.396; p = 0.007 for the mean TBI taken from right side and left side values), as well as the maximal toe pressure (TP) taken from right side and left side values (R = 0.304; p = 0.045). Thyrotropin (TSH) was shown to be significantly correlated only with the maximal TBI value (taken from right side and left side values) (R = 0.318; p = 0.033), whereas free thyroxin (FT4) was shown to be significantly correlated only with the minimal TBI value (taken from right side and left side values) (R = 0.381; p = 0.01). Thyroid volume (TV) was shown to be correlated with TP (R = 0.4; p = 0.008 for the mean TP taken from right side and left side values) and some parameters of TBI value (R = 0.332; p = 0.028 for the mean TBI taken from right side and left side values), but no significant correlations were found between TVand ABI parameters. Patients with a mean ABI value ≤ 1.0 or a mean TBI value ≤ 0.75 have lower TSH, FT3, FT4, and TV than the rest of the study population, but the difference was statistically significant only for FT3. Conclusions: Even in a population of euthyroid patients with no diagnosed thyroid disease, there are some significant correlations between the volume and function of the thyroid gland and the selected features of subclinical cardiovascular dysfunction such as ABI and TBI.

Reduced sensitivity to thyroid hormones is associated with lung function in euthyroid individuals

Background

The thyroid gland exhibits a subtle interconnection with the lungs. We further investigated the correlation between thyroid hormone sensitivity and lung function in euthyroid individuals.

Methods

Data on spirometry and mortality for participants aged 19-79 years were extracted from the NHANES database. Obstructive lung function was defined as a forced expiratory volume in 1 s to forced vital capacity ratio (FEV1/FVC) < 0.70, while restrictive lung function was considered when FEV1/FVC ≥0.70 and baseline FVC <80 % predicted. Central and peripheral sensitivities to thyroid hormones were mainly evaluated by Thyroid Feedback Quantile-based Index (TFQI) and Free Triiodothyronine/Free thyroxine (FT3/FT4) ratio. Logistic regression and subgroup analysis were used to examine potential associations between thyroid hormone sensitivity and lung function. The association between TFQI and all-cause mortality risk was also investigated.

Results

A total of 6539 participants were analyzed, 900 with obstructive lung function and 407 with restrictive lung function. The prevalence of impaired lung function, both obstructive and restrictive, increased with higher TFQI levels. Logistic regression analysis showed that increased TFQI and decreased FT3/FT4 levels were independent risk factors for obstructive and restrictive lung function (P < 0.05). After adjusting for the impact of lung function, TFQI (HR = 1.25, 95 % CI 1.00-1.56, P = 0.048) was an independent risk factor for all-cause mortality.

Conclusion

Reduced sensitivity to thyroid hormones has been linked to impaired lung function. TFQI and FT3/FT4 are potential epidemiological tools to quantify the role of central and peripheral thyroid resistance in lung function.

Causal association of TSH with ischemic heart diseases and heart failure: A 2-sample Mendelian randomization study

Thyroid dysfunction is associated with the risk of cardiovascular disease; however, whether plasma thyroid-stimulating hormone (TSH) levels in subjects with euthyroidism affect the risk of cardiovascular disease remains unclear. This study aimed to investigate the causal association between plasma TSH levels and cardiovascular diseases, particularly ischemic heart disease and heart failure (HF). Summary statistics from the Integrative Epidemiology Unit Open genome-wide association studies Project and FinnGen consortium were used to investigate the causal relationship between plasma TSH levels and the risk of cardiovascular diseases. Two-sample Mendelian randomization analysis using inverse-variance weighting as the primary method was performed. The MR Pleiotropy RESidual Sum and Outlier and leave-one-out methods were used to ensure the robustness of our findings. Genetically determined plasma TSH levels were associated with major coronary heart disease events (OR 1.0557, 95% CI 1.0141-1.0991), all-cause HF (OR 0.9587, 95% CI 0.9231-0.9956), and HF + non-ischemic cardiomyopathy (OR 0.9318, 95% CI 0.8786-0.9882). After the Bonferroni correction, the causation described above disappeared. In the secondary analysis, genetically determined higher TSH levels were associated with a higher risk for unstable angina pectoris (OR 1.0913, 95% CI 1.0350-1.1507), but were associated with a lower risk for HF + overweight (OR 0.9265, 95% CI 0.8821-0.9731). These results were further validated using sensitivity analysis. Our findings show that increased plasma TSH levels in patients with euthyroidism may increase the risk of unstable angina pectoris but reduce the risk of HF in overweight patients. This evidence indicates that plasma TSH levels may need to be carefully controlled in specific patients.

Association of sensitivity to thyroid hormones with all-cause mortality in euthyroid US adults: A nationwide cohort study

Background This study aimed to examine the associations of thyroid hormone sensitivity indices, including free triiodothyronine to free thyroxine (FT3/FT4) ratio, thyroid feedback quantile-based index by FT4 (TFQIFT4), thyroid-stimulating hormone index (TSHI), and thyrotrophic thyroxine resistance index (TT4RI) with all-cause mortality in euthyroid adults. Methods The study included 6243 euthyroid adults from the National Health and Nutrition Examination Survey (NHANES) 2007-2012. FT3/FT4 ratio, TFQIFT4, TSHI, and TT4RI were calculated. The multivariable Cox proportional hazard regression, restricted cubic spline (RCS), and subgroup analysis were conducted. Results Individuals in quartile 4th (Q4) had lower all-cause mortality than those in quartile 1st (Q1) of FT3/FT4 ratio (OR 0.70, 95% CI (0.51, 0.94)). Regarding TFQIFT4, individuals in Q4 of TFQIFT4 had a 43% higher all-cause mortality than those in Q1 (OR 1.43, 95% CI (1.05, 1.96)) (P <0.05, all). Compared with participants in Q1, no associations of TSHI and TT4RI with mortality were found. TFQIFT4 was linearly and positively associated with mortality. However, the FT3/FT4 ratio showed a U-shaped association with mortality. Conclusions Increased risk for all-cause mortality was positively associated with TFQIFT4, suggesting that increased risk for all-cause mortality was associated with decreased central sensitivity to thyroid hormones. Furthermore, the FT3/FT4 ratio showed a U-shaped association with mortality, with an inflection point at 0.5. However, more cohort studies are needed to validate the conclusions.

The different outcomes in the elderly with subclinical hypothyroidism diagnosed by age-specific and non-age-specific TSH reference intervals: a prospectively observational study protocol

Introduction

Subclinical hypothyroidism (SCH) is a common endocrine disorder characterized by elevated thyroid-stimulating hormone (TSH) levels and normal free thyroxine (FT4) levels. The overdiagnosis and overtreatment of SCH in elderly patients have become concerns as TSH levels naturally increase with age. Studies have shown that many elderly patients with SCH can recover without treatment, and the administration of levothyroxine (L-T4) does not improve their prognosis. Therefore, It is necessary to establish age-specific reference ranges for TSH in elderly individuals to aid in clinical decision-making and prevent overdiagnosis.

Methods

This is a multicenter prospective study that focuses on Chinese elderly patients with SCH who have TSH levels below 10 mU/L. After obtaining the informed consent of the patients, their initial diagnosis information will be registered, and they will be asked to fill out questionnaires such as the Montreal Cognitive Assessment-Basic (MoCA-B), Hamilton Depression Scale (HAMD), Hypothyroidism Symptom Questionnaire (SRQ), frail scale(FRAIL), fatigue scale, and EQ-5D. In addition, thyroid function tests, blood lipid analysis, carotid artery ultrasound, and thyroid ultrasound examinations will be conducted. Patients will also be grouped according to FT4 levels, the changes in FT4 and its relationship with TSH can also be described. For patients over 80 years old, a decrease in FT4 will be used as an endpoint event, while for patients between 60-80 years old, TSH levels greater than or equal to 10mIU/L or a decline in FT4 will be used as the endpoint event. The TSH reference intervals of the general and elderly populations will be used to calculate medical costs associated with multiple follow-ups of patients, and a social-economic analysis will also be conducted.

Discussion

This study will prospectively observe elderly patients with SCH who are screened using both age-specific and non-age-specific TSH reference ranges for the elderly population. We will compare the results of elderly patients diagnosed with SCH using different reference ranges and analyze their association with FT4 to identify meaningful SCH patients and reduce over diagnosis and over treatment of elderly SCH.

Ethics

The Medical Science Research Ethics Committee of the First Affiliated Hospital of China Medical University approved this study (ID: AF-SOP-07-1.1-01). The results will be published in an open-access journal.

Clinical trial registration

https://www.chictr.org.cn/, identifier ChiCTR2300070831.

The optimal healthy ranges of thyroid function defined by the risk of cardiovascular disease and mortality: systematic review and individual participant data meta-analysis

BACKGROUND

Reference intervals of thyroid-stimulating hormone (TSH) and free thyroxine (FT4) are statistically defined by the 2·5-97·5th percentiles, without accounting for potential risk of clinical outcomes. We aimed to define the optimal healthy ranges of TSH and FT4 based on the risk of cardiovascular disease and mortality.

METHODS

This systematic review and individual participant data (IPD) meta-analysis identified eligible prospective cohorts through the Thyroid Studies Collaboration, supplemented with a systematic search via Embase, MEDLINE (Ovid), Web of science, the Cochrane Central Register of Controlled Trials, and Google Scholar from Jan 1, 2011, to Feb 12, 2017 with an updated search to Oct 13, 2022 (cohorts found in the second search were not included in the IPD). We included cohorts that collected TSH or FT4, and cardiovascular outcomes or mortality for adults (aged ≥18 years). We excluded cohorts that included solely pregnant women, individuals with overt thyroid diseases, and individuals with cardiovascular disease. We contacted the study investigators of eligible cohorts to provide IPD on demographics, TSH, FT4, thyroid peroxidase antibodies, history of cardiovascular disease and risk factors, medication use, cardiovascular disease events, cardiovascular disease mortality, and all-cause mortality. The primary outcome was a composite outcome including cardiovascular disease events (coronary heart disease, stroke, and heart failure) and all-cause mortality. Secondary outcomes were the separate assessment of cardiovascular disease events, all-cause mortality, and cardiovascular disease mortality. We performed one-step (cohort-stratified Cox models) and two-step (random-effects models) meta-analyses adjusting for age, sex, smoking, systolic blood pressure, diabetes, and total cholesterol. The study was registered with PROSPERO, CRD42017057576.

FINDINGS

We identified 3935 studies, of which 53 cohorts fulfilled the inclusion criteria and 26 cohorts agreed to participate. We included IPD on 134 346 participants with a median age of 59 years (range 18-106) at baseline. There was a J-shaped association of FT4 with the composite outcome and secondary outcomes, with the 20th (median 13·5 pmol/L [IQR 11·2-13·9]) to 40th percentiles (median 14·8 pmol/L [12·3-15·0]) conveying the lowest risk. Compared with the 20-40th percentiles, the age-adjusted and sex-adjusted hazard ratio (HR) for FT4 in the 80-100th percentiles was 1·20 (95% CI 1·11-1·31) for the composite outcome, 1·34 (1·20-1·49) for all-cause mortality, 1·57 (1·31-1·89) for cardiovascular disease mortality, and 1·22 (1·11-1·33) for cardiovascular disease events. In individuals aged 70 years and older, the 10-year absolute risk of composite outcome increased over 5% for women with FT4 greater than the 85th percentile (median 17·6 pmol/L [IQR 15·0-18·3]), and men with FT4 greater than the 75th percentile (16·7 pmol/L [14·0-17·4]). Non-linear associations were identified for TSH, with the 60th (median 1·90 mIU/L [IQR 1·68-2·25]) to 80th percentiles (2·90 mIU/L [2·41-3·32]) associated with the lowest risk of cardiovascular disease and mortality. Compared with the 60-80th percentiles, the age-adjusted and sex-adjusted HR of TSH in the 0-20th percentiles was 1·07 (95% CI 1·02-1·12) for the composite outcome, 1·09 (1·05-1·14) for all-cause mortality, and 1·07 (0·99-1·16) for cardiovascular disease mortality.

INTERPRETATION

There was a J-shaped association of FT4 with cardiovascular disease and mortality. Low concentrations of TSH were associated with a higher risk of all-cause mortality and cardiovascular disease mortality. The 20-40th percentiles of FT4 and the 60-80th percentiles of TSH could represent the optimal healthy ranges of thyroid function based on the risk of cardiovascular disease and mortality, with more than 5% increase of 10-year composite risk identified for FT4 greater than the 85th percentile in women and men older than 70 years. We propose a feasible approach to establish the optimal healthy ranges of thyroid function, allowing for better identification of individuals with a higher risk of thyroid-related outcomes.

FUNDING

None.

TSH levels within the normal range and risk of cardiovascular and all-cause mortality among individuals with diabetes

BACKGROUND

Evidence regarding thyroid-stimulating hormone (TSH) levels within the normal range and mortality in adults with diabetes is scarce. This study aimed to identify the association between TSH levels and cardiovascular disease (CVD) and all-cause mortality among euthyroid patients with diabetes.

METHODS

This prospective cohort study included 1830 adults with diabetes from the Third National Health and Nutrition Examination Survey III. Mortality outcomes were ascertained by linkage to National Death Index records through December 31, 2019. Participants were categorized by tertiles of TSH levels (low-normal, 0.39-1.30 mIU/L; medium-normal, 1.30-2.09 mIU/L; high-normal, 2.09-4.60 mIU/L). Multivariable Cox proportional hazards models were used to explore the association between TSH levels within the normal range and overall and CVD mortality. Furthermore, restricted cubic spline analyses were used to determine the nonlinear relationship between TSH levels and mortality.

RESULTS

During a median follow-up of 17.1 years, 1324 all-cause deaths occurred, including 525 deaths from CVD. After multivariate adjustment, a U-shaped relationship was observed between TSH levels in euthyroid status and all-cause or CVD mortality among patients with diabetes (both P < 0.05 for nonlinearity). Compared with participants with medium-normal TSH levels, those with high-normal TSH levels had a significantly higher risk of all-cause (hazard ratio, 1.31; 95% confidence interval, 1.07-1.61) and CVD (1.52; 1.08-2.12) mortality. Similarly, low-normal TSH levels also increased all-cause (1.39; 1.12-1.73) and CVD (1.69; 1.17-2.44) mortality risk. In stratum-specific analyses, we found that high-normal TSH levels were associated with higher mortality risk in younger (< 60 years) patients with diabetes but not in older (≥ 60 years) participants.

CONCLUSION

Low- and high-normal serum TSH levels were associated with increased all-cause and CVD mortality in euthyroid adults with diabetes. Further studies are needed to confirm the present observation in a wider population.

Minor perturbations of thyroid homeostasis and major cardiovascular endpoints—Physiological mechanisms and clinical evidence

It is well established that thyroid dysfunction is linked to an increased risk of cardiovascular morbidity and mortality. The pleiotropic action of thyroid hormones strongly impacts the cardiovascular system and affects both the generation of the normal heart rhythm and arrhythmia. A meta-analysis of published evidence suggests a positive association of FT4 concentration with major adverse cardiovascular end points (MACE), but this association only partially extends to TSH. The risk for cardiovascular death is increased in both subclinical hypothyroidism and subclinical thyrotoxicosis. Several published studies found associations of TSH and FT4 concentrations, respectively, with major cardiovascular endpoints. Both reduced and elevated TSH concentrations predict the cardiovascular risk, and this association extends to TSH gradients within the reference range. Likewise, increased FT4 concentrations, but high-normal FT4 within its reference range as well, herald a poor outcome. These observations translate to a monotonic and sensitive effect of FT4 and a U-shaped relationship between TSH and cardiovascular risk. Up to now, the pathophysiological mechanism of this complex pattern of association is poorly understood. Integrating the available evidence suggests a dual etiology of elevated FT4 concentration, comprising both ensuing primary hypothyroidism and a raised set point of thyroid function, e. g. in the context of psychiatric disease, chronic stress and type 2 allostatic load. Addressing the association between thyroid homeostasis and cardiovascular diseases from a systems perspective could pave the way to new directions of research and a more personalized approach to the treatment of patients with cardiovascular risk.

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.

FT3/FT4 ratio is correlated with all-cause mortality, cardiovascular mortality, and cardiovascular disease risk: NHANES 2007-2012

BACKGROUND

Thyroid hormones play a vital role in maintaining the homeostasis of the cardiovascular system. The FT3/FT4 ratio can be used to evaluate the rate of T4-to-T3 conversion, reflecting the peripheral sensitivity of thyroid hormones. There is no study to investigate its relationship with death and cardiovascular disease (CVD) in the general population.

METHODS

This retrospective cohort study involved 8,018 participants with measured thyroid function and no prior thyroid disease who participated in the National Health and Nutrition Examination Survey (NHANES) from 2007 to 2012. Mortality status was determined by routine follow-up using the National Death Index through December 31, 2015.

RESULTS

During a median of 87 months of follow-up, we observed 699 all-cause deaths, including 116 cardiovascular deaths. In multivariate adjusted models, higher free thyroxine (FT4) was linked to increased all-cause mortality (HR, 1.15 per SD; 95% CI, 1.09-1.22), cardiovascular mortality (HR, 1.18 per SD; 95% CI, 1.01-1.39), and CVD risk (HR, 1.17 per SD; 95% CI, 1.08-1.27). Higher free triiodothyronine (FT3) was linked to decreased all-cause mortality (HR 0.81 per SD; 95% CI, 0.70-0.93). Higher FT3/FT4 ratio was linked to decreased all-cause mortality (HR, 0.77 per SD; 95% CI, 0.69-0.85), cardiovascular mortality (HR, 0.79 per SD; 95% CI, 0.62-1.00), and CVD risk (HR, 0.82 per SD; 95% CI, 0.74-0.92). The FT3/FT4 ratio stratified findings were broadly consistent with the overall results.

CONCLUSIONS

FT3, FT4, and the FT3/FT4 ratio were all independent predictors of all-cause death. FT4 and the FT3/FT4 ratio, but not FT3, were independent predictors of cardiovascular mortality and CVD risk. Along with FT3 and FT4, we should pay equal attention to the FT3/FT4 ratio in the general population.

Thyroid testing paradigm switch from thyrotropin to thyroid hormones-Future directions and opportunities in clinical medicine and research

PURPOSE

Recently published papers have demonstrated that particularly in untreated individuals, clinical parameters more often associate with thyroid hormone, particularly free thyroxine (FT4), levels than with thyrotropin (TSH) levels. Clinical and research assessments of the thyroid state of peripheral tissues would therefore be more precise if they were based on FT4 levels rather than on TSH levels. In this paper we describe implications of, and opportunities provided by, this discovery.

CONCLUSIONS

The FT4 level may be the best single test of thyroid function. The addition of free triiodothyronine (FT3) and TSH levels would further enhance test sensitivity and distinguish primary from secondary thyroid dysfunction respectively. There are opportunities to reconsider testing algorithms. Additional potential thyroidology research subjects include the peripheral differences between circulating FT4 and FT3 action, and outcomes in patients on thyroid replacement therapy in terms of thyroid hormone levels. Previously performed negative studies of therapy for subclinical thyroid dysfunction could be repeated using thyroid hormone levels rather than TSH levels for subject selection and the monitoring of treatment. Studies of outcomes in older individuals with treatment of high normal FT4 levels, and pregnant women with borderline high or low FT4 levels would appear to be the most likely to show positive results. There are fresh indications to critically re-analyse the physiological rationale for the current preference for TSH levels in the assessment of the thyroid state of the peripheral tissues. There may be opportunities to apply these research principles to analogous parameters in other endocrine systems.