Brain [¹⁸F]Fluorodeoxyglucose Metabolism Assessment under Hypothyroidism and Recombinant Human Thyroid-Stimulating Hormone in Comparison with Thyroid Hormone Replacement in Patients Submitted to Total Thyroidectomy

Abstract

Objective: To compare brain metabolism using [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) positron emission tomography/computed tomography (PET/CT) in total thyroidectomy patients during hypothyroidism (levothyroxine withdrawal) or under recombinant human thyroid-stimulating hormone (rhTSH) against levothyroxine intake. Methods: A total of 12 patients were randomly divided into two groups. One group underwent the first [¹⁸F]FDG PET/CT brain scan after levothyroxine withdrawal (hypothyroidism condition) and repeated the scan 6 months later during regular levothyroxine intake (replacement condition). The other group underwent the first [¹⁸F]FDG PET/CT scan after receiving an rhTSH injection and maintained regular levothyroxine intake (rhTSH condition), and repeated the scan 7 months later during regular levothyroxine intake without rhTSH administration. The intra-group regional brain metabolisms were compared. Results: Under the hypothyroidism condition, brain metabolism was significantly reduced, namely in the bilateral pre-frontal, temporal, anterior cingulate, and primary motor cortices, insula, and striatum (uncorrected voxelwise p < 0.005); No significant differences were found between the rhTSH and replacement conditions. Conclusion: rhTSH administration could be a better option than levothyroxine withdrawal for ¹³¹I treatment, serum thyroglobulin measurement, or radioiodine scanning for patient follow-up.

1. Introduction

Worldwide, thyroid cancer ranks in ninth place for the number of incidences, with a total of 567 thousand cases in 2018 [1]. Partial or total thyroidectomy is recommended in many cases [2]. After total thyroidectomy, patients need regular thyroxine (T4) replacement (levothyroxine) intake. Radioactive iodine therapy with ¹³¹I helps to eliminate the remaining normal thyroid tissue, irradiates the possible neoplastic tissue remnants, and/or treats persistent recurrent disease [3]. To optimize this therapeutic effect, ¹³¹I uptake by thyroid cells and/or thyroid cancer metastases should be maximized, and therefore, the levels of the thyroid-stimulating hormone (TSH) should be as high as possible. This can be achieved by withdrawing levothyroxine 4 to 5 weeks before ¹³¹I treatment [3]. Levothyroxine withdrawal induces hypothyroidism, a serious condition with non-specific symptoms, including mild to moderate weight gain, fatigue, poor concentration, and depression [4].

Short-term hypothyroidism induced by levothyroxine withdrawal, frequently experienced by patients undergoing radioiodine treatment, seriously impacts multiple organs and systems. In severe cases, it can impair the quality of life and may be dangerous for the elderly [5]. It may also exacerbate neuropsychiatric illness [5].

Short-term hypothyroidism’s impact on brain metabolism is not yet totally clear or well understood. A recent functional magnetic resonance imaging (fMRI) study performed after the partial withdrawal of levothyroxine revealed deficits in a working memory task and the activation of brain areas associated with working memory ability, namely in the cerebellum, insula, parietal, frontal, temporal, and occipital lobes, lingual gyrus, and the cuneus [6].

A recent cross-sectional study based on [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) positron emission tomography/computed tomography (PET/CT) detected different brain metabolism patterns between papillary thyroid cancer patients that suspend levothyroxine for four weeks before the [¹⁸F]FDG PET/CT scan and patients that kept their regular levothyroxine intake [7]. Depression and anxiety were also detected in the patients that suspended levothyroxine intake [7].

As an alternative to levothyroxine withdrawal, recombinant human TSH (rhTSH) may be administered in the two days before radioactive iodine therapy [3,5,8,9] or before periodic serum thyroglobulin measurements for disease restaging and radioiodine scanning, with similar treatment efficacy, as induced by hypothyroidism [10].

Although a few studies have focused on understanding the effects of short-term hypometabolism in the brain functioning induced by levothyroxine withdrawal, the effects of rhTSH administration on brain metabolism remain unknown. These are the main motivations of the current study.

Thus, the purpose of this study is to investigate the differences in the brain metabolism between short-term hypothyroidism (levothyroxine withdrawal) and rhTSH stimulation against regular levothyroxine intake in patients submitted to total thyroidectomy. This is assessed based on glucose consumption measured by [¹⁸F]FDG PET/CT in a longitudinal study. In addition, it also aims to investigate the optimal period after [¹⁸F]FDG injection for performing the PET/CT scan, which translates into greater differences when compared to the regular levothyroxine intake condition.

2. Material and Methods

2.1. Study Design

Twelve thyroid carcinoma patients (

Table 1. Patients’ demographics. IQR: interquartile range.

	Sex (Male/Female)	Age at the Time of First [18F]FDG PET/CT Brain Scan (Median (IQR) Years)	Education Level (Median (IQR) Years)
Withdrawal group	1/5	36 (32)	11 (9)
rhTSH group	2/4	45 (10)	8 (9)

) who had previously undergone total thyroidectomy for thyroid cancer were recruited. Participants were excluded if they had neurological or psychiatric disorders or any other condition that might affect baseline brain metabolism.

Patients were randomly divided into two groups of six (Figure 1): the withdrawal group (replacement versus hypothyroidism) and the rhTSH group (replacement versus rhTSH administration). Patients from the withdrawal group suspended the levothyroxine pills intake 3–4 weeks before the first [¹⁸F]FDG PET/CT brain study. Then, they resumed their usual levothyroxine intake. Approximately 6 months after, during the replacement condition, they underwent the second [¹⁸F]FDG PET/CT brain scan. Patients from the rhTSH group were injected with 0.9 mg of rhTSH (Thyrogen^®, Sanofi) in the two days before the first [¹⁸F]FDG PET/CT brain scan while sustaining the usual levothyroxine medication. Approximately 7 months later, they underwent the second [¹⁸F]FDG PET/CT brain scan during the replacement condition.

The neuropsychological evaluation was done immediately before the [¹⁸F]FDG PET/CT brain scans, using the dementia rating scale-2 (DRS-2), trail making test A (TMTA) and B (TMTB), auditory verbal learning test (AVLT), digit span total (DST), hospital anxiety and depression scale for anxiety (HADS-A), and hospital anxiety and depression scale for depression (HADS-D). The DRS-2 assesses a patient’s overall level of cognitive functioning (the higher the score the better); TMTA and TMTB measure the time needed for performing a task (the higher the score the worst); AVLT evaluates verbal memory in patients (the higher the score the better); DST measures the ability to memorize a sequence of numbers (the higher the score the better); HADS-A and HADS-D aim to measure symptoms of anxiety and depression, respectively (the higher the score the worst).

Levothyroxine withdrawal and rhTSH administration were carried out for periodic serum thyroglobulin measurement, with the [¹⁸F]FDG PET/CT brain scans carried out at that time. No patient underwent radioablation between the two [¹⁸F]FDG PET/CT brain studies. Thyroid function was tested on the days of the [¹⁸F]FDG PET/CT brain scans by measuring TSH, free thyroxine T4 (FT4), and triiodothyronine (T3) serum levels.

2.2. [¹⁸F]FDG PET/CT Brain Acquisitions

In all [¹⁸F]FDG PET/CT brain studies, two acquisitions were carried out per patient: approximately 30 min and 2 h post-injection. After the injection of a median activity of 166 MBq, patients were resting in a quiet dimly lit room with their eyes open. They were not required to fast before the [¹⁸F]FDG injection. PET/CT acquisitions were performed on a GE Discovery LS (GE Medical Systems, USA). For PET, a 3D Kinahan-Roger (GE Medical Systems, USA) reconstruction algorithm, with attenuation and scatter corrections, was used. A voxel size of 1.95 × 1.95 × 4.25 mm³ and matrix size of 128 × 128 × 35 were defined. CT images were used for attenuation correction and anatomical referencing, and reconstructed with a voxel size of 0.98 × 0.98 × 4.25 mm³ and matrix size of 512 × 512 × 35.

2.3. Image Processing

For each patient, all four [¹⁸F]FDG PET brain volumetric images were rigidly aligned (registered) with each other. Then, the average image was taken and registered to the Montreal Neurological Institute (MNI) space (ICBM 2009a Nonlinear Symmetric T1) using a suitable deformable model. The geometric deformation model found was then applied to all four images already aligned with each other, which were resampled with an isotropic voxel size of 2 mm afterwards. After this, the resampled images were smoothed with a Gaussian kernel with 12 mm full width at half maximum, and finally intensity normalized. This intensity normalization was carried out at the voxel level (voxelwise), independently, per image, using the pons as the reference region, i.e., each voxel standardized uptake value (SUV) was divided by the mean pons SUV, originating a voxelwise SUV ratio (SUVR). The SUV is a voxelwise semi-quantitative scale, usually automatically quantified by the PET scanners. In this work it was computed as:

$$\mathrm{SUV}=\frac{\mathrm{activity}\mathrm{concentration}\left(\frac{\mathrm{Bq}}{\mathrm{mL}}\right)}{\mathrm{injected}\mathrm{activity}\left(\mathrm{Bq}\right)}\times \mathrm{body}\mathrm{weight}\left(\mathrm{g}\right)$$

(1)

2.4. Statistical Analysis

Voxelwise whole-brain statistical analysis was performed on statistical parametric mapping 12 (SPM12) (Wellcome Trust Centre for Neuroimaging) using paired t-tests in the withdrawal group, between the replacement and hypothyroidism conditions, and in the rhTSH group, between the replacement and rhTSH conditions. All analyses were carried out independently for the 30 min and 2 h post-injection acquisitions. Differences were considered statistically significant for p-values < 0.005 (voxelwise uncorrected p-value) due to the exploratory nature of this study.

Additionally, for an uptake evolution analysis over time, the ratios between mean brain SUV at 30 min and 2 h post-injection (SUV_{2 h}/SUV_{30 min}) were computed, and intra-group compared.

3. Results

3.1. Neuropsychological Assessment of Thyroid Function

There were no statistically significant differences in the neuropsychological assessment scores between the replacement and hypothyroidism conditions and between replacement and rhTSH conditions (

Table 2. Patients’ neuropsychological ratings (median (IQR)).

Group	Condition	DRS-2	TMTA	TMTB	AVLT	DST	HADS-A	HADS-D
Withdrawal	Hypothy.	132 (24)	43 (75)	88 (66) *	45 (8)	8 (3)	6 (7)	3 (4)
	Replace.	132 (15)	36 (83)	59 (18) *	46 (8)	10 (4)	6 (5)	2 (3)
rhTSH	rhTSH	130 (17)	52 (28)	54 (126) *	46 (19)	10 (3)	11 (10)	8 (6)
	Replace.	137 (20)	52 (47)	40 **	48 (20)	11 (3)	12 (10)	5 (8)

* Two missing data; ** three missing data, IQR was not computed.

). Cut-offs were applied in all neuropsychological tests after adjusting to the Portuguese population, age, and education level. The hypothyroidism patients group did not reveal more cognitive deficits than the others. However, three patients showed deficits in their general mental state (DRS-2), anxiety (HADS-A), memory (AVLT), and executive functions (TMTB) under the hypothyroidism condition, which was not evident for the replacement condition.

Table 3. TSH, FT4, and T3 serum concentrations under the different conditions (median (IQR)). TSH is in mU/L, FT4 in pmol/dL and T3 in nmol/L. * Three missing data, IQR was not computed.

Group	Condition	TSH	FT4	T3
Withdrawal	Hypothy.	133 (78.4)	4.25 (2.59)	0.61 (0.61)
	Replace.	0.02 (0.02)	22.23 (2.92)	2.17 (0.45)
rhTSH	rhTSH	20.0 (58.6)	19.47 (6.65)	1.78 (0.22)
	Replace.	0.19 (0.39)	17.21 *	1.70 (0.82)

presents the TSH, FT4, and T3 serum concentrations on the days of the PET scans.

3.2. Brain Metabolism

3.2.1. Hypothyroidism versus Replacement Conditions

The Brain [¹⁸F]FDG SUVR was significantly lower during the hypothyroidism condition when compared to the replacement condition in large areas of the brain, especially in the 2 h post-injection acquisitions (Figure 2). There were no brain areas with significantly higher SUVR in the hypothyroidism condition than in the replacement condition.

3.2.2. rhTSH Administration versus Replacement Condition

Brain SUVR was not significantly higher under rhTSH administration compared to the replacement condition, except for the 30 min post-injection acquisitions in very small regions of possibly limited value, taking into account the small number of patients included in this study. No brain region showed statistically higher SUVR during replacement condition when compared to the rhTSH condition.

3.2.3. [¹⁸F]FDG Brain Uptake over Time

Mean brain SUV decreased significantly (Wilcoxon signed ranks test 2-tailed exact p = 0.031) from 30 min to 2 h post-injection for all patients in all conditions (SUV_{2 h}/SUV_{30 min} < 1, Figure 3). It seems there is a trend for a higher ratio under the hypothyroidism condition than under the replacement condition and a lower ratio under rhTSH stimulation than under the replacement condition; however, the differences are not statistically significant (Wilcoxon signed ranks test 2-tailed exact p = 0.156 and p = 0.063, respectively).

4. Discussion

Brain metabolism was significantly lower during hypothyroidism than during the replacement condition in the large cortical and subcortical brain structures. This suggests that levothyroxine withdrawal negatively influences brain metabolism in critical areas related to affect, emotion, memory, and motor activity, which agrees with previous studies that examined the relationship between brain metabolism and hypothyroidism [11,12,13].

rhTSH administration did not reveal significant differences in metabolism, which suggests that brain metabolism during rhTSH is not significantly different from the one during replacement.

Regarding the time evolution analysis of SUV (between 30 min and 2 h post-injection acquisitions), the results showed a statistically significant reduction in all conditions from the 30 min acquisition to the 2 h acquisition. No statistically significant differences were found in the behavior among the conditions.

To the best of the author’s knowledge, this is the first and only intra-subject study comparing [¹⁸F]FDG brain uptake in patients during hypothyroidism conditions or rhTSH administration against replacement conditions at different times post-injection. This allowed us to obtain small differences that might be related to some unrelated changes between cerebral blood flow/blood-pool/perfusion and brain metabolism under study conditions.

Unfortunately, the interpretation of neuropsychological tests was hampered by the small number and heterogeneity of the sample assessed. We would not expect any significant differences between the rhTSH and replacement conditions; however, we envisaged finding worse scores during hypothyroidism when compared to the replacement condition. The relatively short-term hypothyroidism condition may further explain the lack of neuropsychological score differences.

A previous study has also compared brain metabolism after levothyroxine withdrawal against regular levothyroxine replacement [7]. Another study compared levothyroxine withdrawal against rhTSH stimulation [14]. In both cases, changes in brain metabolism were found after levothyroxine withdrawal. Compared with these studies, the present study has the advantage of a within-subject comparison with brain metabolism measurement at different times post-injection.

Difference between-subjects levothyroxine doses were not considered since the within-subject study design prevented the dose from influencing the results. In addition, we were not interested in studying the correlation between levothyroxine dose and [¹⁸F]FDG brain uptake.

The main limitations of the present study are:

(a)

The small number of patients in the sample, which very likely does not represent the full spectrum of patients undergoing total thyroidectomy. Still, this dataset is unique and extremely valuable and was very difficult to obtain mainly due to ethical reasons. The patients included in this study had to undergo two [¹⁸F]FDG PET/CT studies exclusively for this study, which implies a significant ionizing radiation exposition. Note that the patients receive radiation from the [¹⁸F]FDG radiopharmaceutical twice and from four CT scans (two scans per [¹⁸F]FDG injection);

(b)

Even though the intensity normalization by the mean pons SUV is relatively common and is a better option than other central nervous system regions for normalization [15,16], we cannot exclude that the pons may be influenced anyway by the conditions herein under investigation. While other regions could have been considered for metabolism normalization, in a previous study on breast cancer patients, we tested several regions for brain metabolism normalization and concluded that the pons were the best of the ones tested [16]. Although testing different reference regions was not our goal in this work, we also carried out similar analyses using the cerebellum, cerebellar peduncles, and entire brain as reference regions (see Supplementary Figure S1). The results obtained showed a similar tendency as those obtained using the pons as a reference region, but with a lesser size effect;

(c)

Patients were not asked to fast before the [¹⁸F]FDG PET/CT brain scans, and this may have influenced the [¹⁸F]FDG brain SUV [17]; however, the uptake normalization carried out here minimizes the variability which could have been observed if the simple SUV was used.

5. Conclusions

The data herein presented shows that there is no evidence that rhTSH administration significantly alters brain metabolism and functioning, while levothyroxine withdrawal does reduce brain metabolism. Thus, in terms of brain metabolism and functioning, rhTSH administration may be a better option than levothyroxine withdrawal in the preparation of patients for ¹³¹I treatment, serum thyroglobulin measurements, or radioiodine scanning for patient follow-up.