Triiodothyronine Levels in Athyreotic Individuals During Levothyroxine Therapy FREE

The output of the human thyroid gland provides 15% to 20% of circulating triiodothyronine (T₃). Nevertheless, thyroxine (T₄) therapy, using synthetic levothyroxine (LT₄), is the standard of care for thyroid hormone therapy in patients with hypothyroidism. The regulated peripheral conversion of LT₄ to T₃ in humans has previously been demonstrated.^1- 4 Such conversion thus makes it possible to achieve normal T₃ levels in humans treated with LT₄,⁵ albeit with the necessity for maintaining T₄ levels at the higher end of the normal range.^6- 13 However, prior studies have not compared T₃ levels on LT₄ with levels previously observed in euthyroid patients serving as their own control, therefore not addressing the question of whether individuals have deficient T₃ concentrations based on their own particular thyroid axis set point.

Despite documentation of LT₄ to T₃ conversion, there has long been interest in combining T₃ therapy with LT₄, based on the premise that T₃ levels may be lower or inappropriately balanced in patients treated with LT₄. It has been suggested that this putative T₃ deficiency may be associated with failure to fully reverse the symptoms of hypothyroidism.^14- 15 Residual symptoms associated with LT₄ monotherapy have been hypothesized to include cognitive impairment, depression, and decreased psychomotor performance.^14,16- 18 A recent study did document decrements in psychological function, working memory, and motor learning in euthyroid patients treated with LT₄.¹² However, these patients also had higher thyroid-stimulating hormone (TSH) values than the euthyroid control patients.

Many diverse human studies have been unable to demonstrate a consistent, objective benefit of T₃ combination therapy, despite initial studies suggesting improved mood and short-term memory with T₃ supplementation.^19- 20 These studies have included randomized placebo-controlled studies,^21- 24 randomized parallel-design studies,^25- 26 crossover studies,^{19- 20,27- 32} studies focusing on patients with both hypothyroidism and depressive symptoms,^22- 23 a study of patients with thyroid cancer,³² and recent meta-analyses.^15,33 Some of these studies have flaws, such as production of iatrogenic hyperthyroidism with therapy,^{19,25,27- 28} small sample size,^{20,27- 28,30} different TSH levels in treatment groups,^29- 30 failure to use athyreotic patients,^22,25,27 and failure to replicate the normal molar ratio of T₄:T₃.^{19,21- 23,25,29} Additional problems have included lack of placebo control^20,25,27 and improvement in symptoms in the placebo group.^21,24,28

The inability to confirm that combined LT₄ and T₃ therapy is beneficial could potentially be interpreted in many different ways. It is possible that the T₃ doses used have been too high and too infrequently administered, or that sustained-release T₃ is necessary to simulate normal physiology. It is also possible that assessment tools lack the necessary sensitivity to detect subtle improvements in mood, cognitive functioning, and performance. Of interest, in 4 of these studies,^20,25,27,30 the combination therapy was preferred by patients, despite the lack of a demonstrable benefit. In 3 other studies, there was either no preference^29,32 or LT₄ was the preferred therapy.³¹

This study was designed to compare the circulating levels of T₄ and T₃ produced by the normally functioning thyroid gland with those levels resulting from standard thyroid hormone therapy in the same patient. Thyroid hormone levels were measured before thyroidectomy when individuals were not receiving thyroid hormone, and again after surgery when they were stabilized while receiving LT₄ therapy. We sought to document whether LT₄ therapy resulted in lower serum T₃ concentrations within individual patients. The null hypothesis was that T₃ levels in patients rendered euthyroid with LT₄ replacement would not be deficient. The study contained 2 subgroups of participants. Some patients were administered replacement LT₄ therapy, with their doses adjusted to keep their serum TSH level in the normal range; and others were administered suppression therapy because of a diagnosis of thyroid cancer, with their LT₄ doses adjusted to achieve a TSH level below the normal range.

The study was approved by the Georgetown University Institutional Review Board and performed in the General Clinical Research Center (GCRC) of Georgetown University Medical Center, Washington, DC. At the first study visit, written informed consent was obtained and participants' medications were then reviewed. Patient age, sex, and self-reported race and ethnicity were recorded according to GCRC protocol. A medical history was obtained and a physical examination was performed. Recorded parameters included weight, blood pressure, heart rate, thyroid examination, and neurological examination. Participants had 2 separate thyroid profiles drawn before their thyroidectomy. Two further thyroid profiles were obtained after thyroidectomy and stabilization while receiving LT₄ replacement therapy or suppression therapy. The medication history and physical examination were repeated at the end of the study. The sequence of events for the study is depicted in Figure 1. Patients with thyroid cancer had a delayed progression through the study if they required radioactive iodine treatment. Patients participated in the study for approximately 13 to 25 weeks, depending on their diagnosis following thyroidectomy. The study was conducted over a 42-month period. Enrollment was continuous during this period, with each study participant proceeding independently through the study.

Participants of both sexes aged 18 to 65 years were recruited from patients referred to the Department of Otolaryngology-Head and Neck Surgery, Georgetown University Medical Center, Washington, DC, for total or near-total thyroidectomy for goiter, nodular thyroid disease, suspected thyroid cancer, or known thyroid cancer. Patients who had a current or previous diagnosis of hyperthyroidism or hypothyroidism were excluded from the study. Patients who were expected to undergo thyroid lobectomy alone were not included in the study. Any patients who were expected to undergo total or near-total thyroidectomy but whose final surgery consisted of lobectomy were withdrawn from the study. Patients who had changes in their estrogen/progesterone therapy, oral contraceptives, or other medications known to affect thyroid hormone protein binding in the 6 weeks before the study were also excluded. Pregnant or lactating patients were not eligible. Patients with chronic, serious diseases such as cardiac, pulmonary, and renal disease or who were currently taking corticosteroid therapy were not eligible for study participation. Patients whose preoperative (first or second) thyroid profiles were abnormal were excluded from the study.

Each thyroid profile consisted of a serum TSH level, free thyroxine (FT₄), and total T₃. Most presurgical profiles were obtained 1 week and 1 day before thyroidectomy. Postsurgical thyroid profiles were usually drawn 8 and 16 weeks after thyroidectomy. These blood tests were drawn between 8:00 AM and 10:30 AM in a fasting state. The LT₄ administration was delayed until after phlebotomy for the postthyroidectomy sampling to obtain trough levels of thyroid hormones. Two separate presurgical profiles were included for each patient to minimize the effect of day-to-day fluctuations in thyroid hormone concentrations, TSH levels, and laboratory assays. Two postsurgical profiles were performed for the same reason, and also because of the likelihood that the initially selected dose of LT₄ would not achieve the desired TSH level. All thyroid profiles were drawn in the GCRC and analyzed by Quest Diagnostics (Madison, New Jersey), LabCorp (Burlington, North Carolina), or Georgetown University Laboratories (Washington, DC). All 4 thyroid profiles for each patient were analyzed by the same laboratory to minimize interassay variation. Samples were not analyzed in batches because of the long duration of the study, the need to adjust thyroid hormone dosages based on thyroid blood test results, and the fact that each study participant was proceeding independently through the study. In addition, an aliquot of each blood sample was analyzed for T₃ measured by liquid chromatography tandem mass spectrometry.

Thyroid-stimulating hormone levels were measured using a third-generation ultrasensitive immunochemiluminometric assay, with a sensitivity of 0.01 mIU/L (laboratory reference ranges, approximately 0.4-4.5 mIU/L). The FT₄ and T₃ levels were measured by chemiluminescent immunoassays. During the study period, reference ranges were approximately 0.80 to 1.80 ng/dL for FT₄ (to convert to pmol/L, multiply by 12.871) and 100 to 220 ng/dL for T₃ (to convert to nmol/L, multiply by 0.0154). Details of the liquid chromatography tandem mass spectrometry assay were previously described.³⁴ Anthropometric measures included weight and height. Weight was measured using a DS 504 Ohaus scale (Ohaus Corporation, Pine Brook, New Jersey); height measurements used an Accustat Stadiometer (Genentech, San Francisco, California). Physiological measurements were performed by standard procedures. Blood pressure and pulse rate were recorded after the patient had been in a seated position for at least 5 minutes. Medication history, dietary history, and medical history were obtained and recorded by using standardized case report forms.

Thyroidectomy was performed at Georgetown University Hospital, Washington, DC, by a head and neck surgeon experienced in performing thyroid surgery. Any perioperative decisions regarding the extent of thyroid surgery were made by the patient's referring physician and surgeon, without regard for study involvement. Postoperative surgical care, treatment of hypocalcemia, and management of any other postoperative complications were uninfluenced by study participation.

Study participants were all prescribed a name brand of LT₄. The particular brand was noted and adherence to the branded product was verified throughout the study. However, not all patients were taking the same brand name. Patients were asked to separate the time of ingestion of multivitamins or calcium supplements from the time of ingestion of their LT₄ by at least 2 hours and to take their LT₄ at least 60 minutes before breakfast. Participants were not permitted to take any T₃-containing products. Use of Armour Thyroid (a naturally derived thyroid replacement containing both T₄ and T₃), liotrix tablets (Thyrolar), or any other T₃-containing thyroid hormone preparation was grounds for discontinuation from the study. The only exception was the group of patients with thyroid cancer who were temporarily taking T₃ (liothyronine sodium [Cytomel]) as part of their preparation for radioiodine scanning and treatment. Patients with thyroid cancer who received liothyronine sodium therapy had not been taking liothyronine sodium for at least 6 weeks before their first postoperative thyroid profile. Postoperative LT₄ dosage was determined by the patient's pathological diagnosis. Patients with benign disease were initially administered 1.7 μg/kg of LT₄ daily with the goal of achieving a TSH level in the lower two-thirds of the normal range. Patients with thyroid cancer were administered 2.2 μg/kg of LT₄ daily with the goal of achieving a subnormal or suppressed TSH level. Absolute body weight, not ideal body weight, was used to calculate dosage. The LT₄ dosage adjustments were made between the 2 postoperative (third and fourth) thyroid profiles to achieve these goals.

Our prospective study involved 2 cohorts of patients undergoing thyroidectomy. The 2 cohorts were patients with benign and malignant thyroid disease and therefore taking replacement or suppressive doses of LT₄, respectively. Interventions were performed between time points 2 and 3 (thyroidectomy and LT₄ initiation) and between time points 3 and 4 (LT₄ adjustment). Each patient served as his or her own control, thereby reducing the influence of intersubject variability in T₃ and FT₄ levels. Statistical services were provided by the GCRC biostatistics core using SAS version 9.1 (SAS Institute Inc, Cary, North Carolina). P<.05 was considered statistically significant. Statistical analysis showed that a sample size of 40 patients would be sufficient to detect a difference of 6 ng/dL between preoperative and postoperative T₃ levels for individual patients with 90% power and α = .05. Data for this calculation were generated from cross-sectional chart review examining the variation between T₃ levels in individual patients. Ultimately, a sample size of 50 patients was used.

Statistical comparison was performed to determine differences in FT₄ and T₃ levels within individuals before and during thyroid hormone therapy. The null hypothesis was that LT₄ therapy did not result in any alteration of T₃ levels. The FT₄ and T₃ levels before and after thyroid hormone therapy were compared using the Wilcoxon signed rank test to determine whether patients had different thyroid hormone levels before and after their thyroidectomy. This nonparametric test was used for analysis because the data were not normally distributed. A Bonferroni correction for multiple comparisons was used. Data were also analyzed using repeated measures analysis of variance, including a random effect of the individual in the repeated measures model. The post hoc covariates considered were sex (male, female), gonadal status (premenopausal, menopausal), age, race/ethnicity (black, white, Asian, Hispanic), laboratory performing the immunoassay (LabCorp, Quest Diagnostics, Georgetown University Laboratories), surgeon (B.D. or other surgeon), initial LT₄ dose, brand of LT₄ (brand name 1 or brand name 2), and year of entry into the study (2004, 2005, 2006, 2007). Candidate covariate testing was performed by using analysis of variance to detect any signal of covariance, although numbers were inadequate for many of these covariates.

Our study was conducted between January 30, 2004, and June 20, 2007. A total of 142 euthyroid patients were approached to recruit the 50 patients who completed the study. Reasons for declining study participation were time demands of the study, inconvenience of study visits, and lack of interest in research studies. Three patients were withdrawn from the study after initiation. One patient was scheduled for a total thyroidectomy for a substernal goiter, but the final surgery was a lobectomy due to concern about recurrent laryngeal nerve injury. An additional patient scheduled for thyroidectomy for a large isthmus nodule instead underwent an isthmusectomy. A final patient with a multinodular goiter decided against pursuing thyroidectomy the day before surgery. Fifty patients completed the study and provided each of the 4 required blood samples. No patients were lost to follow-up. Patients in the study were operated on by 1 of 5 surgeons. However, 78% of the thyroidectomies were performed by 1 surgeon (B.D.). There were no cases of permanent postoperative hypoparathyroidism. However, 1 patient had a unilateral recurrent laryngeal nerve injury requiring speech therapy. There were no adverse events such as phlebotomy-related injuries, apparent allergies to LT₄ excipients, or clinical signs of overt hypothyroidism or hyperthyroidism associated with study participation.

The characteristics of the study patients are shown in Table 1. Thirty-seven patients (74%) were female and the mean age of study participants was 49 years. A total of 34 participants (68%) were white. Seventeen patients (34%) had a diagnosis of thyroid cancer. Thirty-four patients (68%) required an alteration in their LT₄ dose based on the results of their first postoperative thyroid profile.

There were no differences in blood pressure, pulse rate, deep tendon reflex relaxation, or any other physical examination findings, other than the absence of palpable thyroid tissue, in postoperative patients compared with their preoperative state. Table 2 shows the mean TSH, FT₄, and T₃ concentrations during the study. Time points 1 and 2 were prethyroidectomy (the mean of these 2 time points is displayed) and time points 3 and 4 were 2 successive postthyroidectomy assessments. All 4 time points are also displayed individually using box and whisker plots in Figure 2. Data are separated according to whether the patients had a diagnosis of benign thyroid disease or thyroid cancer.

Serum TSH levels were significantly higher than prethyroidectomy values at time point 3, but not at time point 4 (Table 2 and Figure 2). Serum FT₄ levels increased significantly at postthyroidectomy time points 3 and 4 compared with the prethyroidectomy time points 1 and 2 (Table 2 and Figure 2). However, serum FT₄ levels did not differ between time points 3 and 4. Serum T₃ levels were lower at time point 3 but were fully recovered by time point 4 (Table 2 and Figure 2). Hence, the adjustment of LT₄ dosage performed after the third set of thyroid function tests to achieve goal serum TSH levels was associated with normalization of T₃ levels. The increase in FT₄ concentration associated with LT₄ therapy was well illustrated by the significantly increased ratio of FT₄ to T₃ that was observed in patients after they had been transitioned to LT₄ therapy (Table 2).

Serum T₃ levels for each individual patient during the study period are shown in Figure 3 for patients with benign thyroid disease and thyroid cancer, respectively. This plot displays the mean preoperative T₃ value and the postoperative T₃ value at time point 4 as a separate line for each patient. Time point 3 is not displayed in this plot, because this was primarily an adjustment or transition point. These plots also illustrate the intrasubject and intersubject variability in T₃ levels. Preoperative T₃ values within individuals can be observed to be both higher and lower than their postoperative T₃ values.

After thyroid surgery, patients had different TSH levels, both because of the impact of the calculated LT₄ dose on that particular patient and also because of the use of TSH suppression as a management tool for patients with thyroid cancer. A diagnosis of thyroid cancer was associated with lower TSH levels at time points 3 and 4 (P = .003 and P = .002, respectively), higher FT₄ values at time points 3 and 4 (P = .006 and P = .01, respectively), and a higher T₃ concentration measured by liquid chromatography tandem mass spectrometry at time point 4 (P = .04) than a diagnosis of hypothyroidism. Our conclusions did not appear to be altered by any of the remaining covariates used in the analyses. However, these categories were developed post hoc and, in many cases, there were insufficient patients to examine the effect of the covariate separately. However, study conclusions were affected by the serum TSH level achieved with LT₄ therapy.

Despite the use of a weight-based calculation to determine initial LT₄ dose, widely differing serum TSH values were achieved postoperatively (Figure 2 and Figure 4). Postthyroidectomy time points were divided into 3 groupings: those with TSH values more than 4.5 mIU/L, those with serum TSH values between 0.35 and 4.5 mIU/L, and those with TSH values less than 0.35 mIU/L (Figure 4). The mean T₃ concentration associated with each of these groups is shown for immunoassay and liquid chromatography tandem mass spectrometry measurements, respectively. A significantly lower mean T₃ was observed to be associated with the subset of postoperative TSH values more than 4.5 mIU/L, regardless of whether T₃ was measured by immunoassay or liquid chromatography tandem mass spectrometry. If postoperative TSH values were 0.35 to 4.5 mIU/L, the associated T₃ levels were not lower than the entire group. The TSH values less than 0.35 mIU/L were not associated with higher postoperative T₃ values.

Our study was unable to document any deficiency of T₃ in athyreotic patients treated with LT₄ replacement therapy. After adequate adjustment of their LT₄ doses, these patients had serum T₃ levels after thyroidectomy that were not significantly different from their T₃ levels before thyroidectomy. Serum T₃ concentrations were lower in patients who were taking inadequate LT₄ doses. This phenomenon was observed most clearly at the postthyroidectomy time point 3 (Figure 2 and Figure 4). Serum TSH concentrations of 4.5 mIU/L or more were associated with lower T₃ levels than TSH concentrations of 0.35 to 4.5 mIU/L. There was a lower mean T₃ value associated with TSH values of more than 4.5 mIU/L using liquid chromatography tandem mass spectrometry measurements compared with immunoassay measurements. Liquid chromatography tandem mass spectrometry is generally known to be a more specific assay³⁴ and, at least in the case of FT₄ measurement, agrees better with equilibrium dialysis than immunoassay.³⁵ This is the first study to document T₃ sufficiency using patients treated with LT₄ as their own controls rather than using a nonidentical euthyroid control group. Although this information does not directly address the issue of whether patients might feel better taking combination therapy, it does suggest that such therapy is not necessary to replicate normal serum T₃ levels.

Clearly, our study simply documents circulating levels of thyroid hormones and does not measure thyroid gland hormone production or actual tissue levels. With respect to thyroid hormone tissue concentrations, a 1996 study³⁶ showed that tissue T₃ levels were lower in rats treated with LT₄ than in rats treated with combination therapy. We cannot exclude the possibility that cerebral T₃ levels were lower in our participants during their LT₄ therapy. However, there is no a priori reason to suspect this, given that serum T₃ levels remained the same. It could, admittedly, be hypothesized that cerebral production of T₃ by deiodinases could be down-regulated by the high-circulating FT₄ levels associated with the LT₄-treated state. In this prior animal study, a dose of LT₄ that normalized serum TSH was not studied. However, an earlier study³⁷ did demonstrate in rats that a higher dose of LT₄ that produced both normal serum TSH and normal serum T₃ concentrations could be used. If a higher dose of LT₄ had been unable to normalize serum TSH and serum T₃ in rats, this would be in contrast with the results from our study. The possibility remains that interspecies differences, such as the greater magnitude of thyroidal T₃ production in rats, would allow LT₄ therapy to produce tissue euthyroidism in humans but not rodents.

The FT₄ or T₄ levels have been demonstrated to be higher in euthyroid patients treated with LT₄ than in euthyroid controls in previous studies in humans.^6- 13 In our study, we found that within the same patient, FT₄ levels were higher after thyroidectomy than before thyroidectomy. Our data confirm that higher FT₄ levels are a characteristic of LT₄ monotherapy. Furthermore, we postulate based on our data that this is necessary to normalize TSH and serum T₃ levels and thereby achieve euthyroidism. Whether there are adverse consequences of increased FT₄ levels, even when they are not accompanied by decreased serum TSH concentrations, remains to be investigated in future studies.

A potential limitation of our study is that the patients with benign thyroid disease had varying amounts of remnant thyroid tissue. However, it would seem unlikely that such thyroid remnants would function sufficiently to mask the ability to detect a decrement in T₃ concentrations with LT₄ replacement. Analysis included diagnosis as a covariate and patients with thyroid cancer, all of whom underwent remnant ablation with radioactive iodine, did not have lower postoperative T₃ levels compared with patients with hypothyroidism. This suggests that residual thyroid tissue was not contributing significantly to maintenance of circulating T₃ levels in patients with benign disease. This is in contrast with the findings from a previous study²⁰ in which some benefits of T₃ combination therapy were noted in the subgroup of patients with a diagnosis of thyroid cancer. However, patients with thyroid cancer were compared with those with Hashimoto's hypothyroidism, whereas all our study participants had surgically induced hypothyroidism. Admittedly, another variable of a lower serum TSH level was also present in our patients with thyroid cancer. Subgroup analysis, however, showed that having a TSH level suppressed to less than 0.35 mIU/L did not raise postoperative T₃ levels beyond those observed in the group with TSH levels of 0.35 to 4.5 mIU/L (Figure 4).

Another potential limitation of our study is that it had a long duration. It is possible that temporal changes in variables such as laboratory assays, surgeon procedures, LT₄ formulation, and other factors could increase the variation in the measured hormones and thereby decrease the ability to detect changes in T₃ levels. However, including the year of entry into the study as a covariate did not alter study conclusions.

Our study did not examine satisfaction with LT₄ therapy. It was thought to be impossible to discern whether symptoms developing since LT₄ initiation were due to recent major surgery, thyroid cancer treatment, or the LT₄ therapy itself. However, we were unable to demonstrate different serum T₃ levels in patients treated with LT₄ therapy, when their therapy had been adjusted to achieve their target TSH levels, or when their TSH levels were less than 4.5 mIU/L. Based on this fact, there is no reason to hypothesize that patient dissatisfaction with LT₄ treatment is due to inability to achieve the same serum T₃ concentrations that are present in individuals with normal endogenous thyroid function. Certainly, declines in psychological and cognitive performance have been documented in euthyroid patients treated with LT₄, even though as a group these patients had a mean TSH level of 2.6 mIU/L.¹² Possibly, there is some characteristic of LT₄ therapy that does not fully replicate an important but as yet unidentified aspect of normal thyroid physiology. For example, it is possible that there is some differential regulation of and thereby different fluctuation in T₃ levels in endogenous function that is not replicated with LT₄ replacement. There might, therefore, be some use in evaluating patient satisfaction, psychological functioning, and motor performance during therapy with a sustained-release T₃ preparation, which might more closely replicate normal physiology. Patients receiving LT₄ therapy also clearly have higher FT₄ / T₃ ratios than the ratios that were characteristic of their period of endogenous thyroid function. It is also possible that the high concentration of FT₄ that is necessary for normalization of T₃ levels during LT₄ therapy is associated by some unappreciated mechanism with an adverse impact on patients.

Circulating T₃ levels have been shown to be lower in individuals carrying certain deiodinase polymorphisms in some studies.^38- 39 A subgroup of patients who remain dissatisfied with their LT₄ therapy may be individuals who carry a specific deiodinase polymorphism. It is possible that combination LT₄-T₃ therapy may be beneficial in these rare cases. We postulate that these deiodinase polymorphisms were probably not present in our small 50-patient sample, although some outlier T₃ values were observed (Figure 3). Only the group of patients with TSH levels higher than 4.5 mIU/L had T₃ levels lower than other groups of patients treated with LT₄. It would be more logical to assume that the T₃ deficiency in this group was caused by suboptimum LT₄ therapy rather than the presence of a deiodinase polymorphism.

Based on our study results, it would appear reasonable to advise individual patients that physiological T₃ levels can indeed be replicated with LT₄ therapy. If it is assumed that maintenance of normal T₃ concentrations correlates with satisfactory replacement therapy, our results could provide one possible explanation for the failure of multiple studies to demonstrate a benefit of T₃ combination therapy. Our study, however, is limited by the fact that we did not document patients' symptoms. If adequate serum T₃ levels were also correlated with patient satisfaction or well-being, this would support the commonly held belief that LT₄ should remain the standard therapy for hypothyroidism and thyroid cancer.