The Role of 18F-FDG PET/CT in the Management of the Autoimmune Thyroid Diseases

Autoimmune Thyroid Diseases

Autoimmune thyroid diseases (AITDs) include a wide spectrum of thyroid diseases affecting more commonly women than men. The most frequent forms are Graves' Disease (GD) and Hashimoto's thyroiditis / Autoimmune Thyroiditis (AIT), but there are also other immunogenic destructive forms of thyroiditis, that is, silent and postpartum thyroiditis. In the last decade, AITDs and other inflammatory thyroid diseases related to anti-tumor molecular drugs are more frequently seen due to the widespread use of tyrosine kinase inhibitors (TKIs) and immune checkpoint inhibitors (ICPIs). Autoimmune thyroiditis related to SARS-CoV-2 infection has been a novel entity in recent years. Graves' Disease and AIT may shift from hyperthyroidism to hypothyroidism, which may complicate the differential diagnosis and further treatment strategy. Moreover, all AITDs may manifest with thyrotoxicosis (a clinical condition marked with high serum levels of thyroid hormones) which has to be distinguished from hyperthyroidism (increased thyroid hormone production and secretion as a result of hyperfunctioning thyroid gland) due to different therapeutic approaches. Nuclear medicine techniques, such as radioiodine uptake (RAIU) and thyroid scintigraphy, using 99mTc- pertechnetate (Na[99mTc]TcO4) or 123-Iodine (Na[123I]I), have a crucial role in the differential diagnosis. Measurement of thyroid antibodies, e.g. thyroid peroxidase antibodies (TPO) and thyrotropin receptor antibodies (TRAb), as well as thyroid ultrasound, are complementary methods in the evaluation of thyroid disorders.

Significance of FDG-PET standardized uptake values in predicting thyroid disease

OBJECTIVE

This study aimed to determine a standardized cut-off value for abnormal 18F-fluorodeoxyglucose (FDG) accumulation in the thyroid gland.

METHODS

Herein, 7013 FDG-PET/CT scans were included. An automatic thyroid segmentation method using two U-nets (2D- and 3D-U-net) was constructed; mean FDG standardized uptake value (SUV), CT value, and volume of the thyroid gland were obtained from each participant. The values were categorized by thyroid function into three groups based on serum thyroid-stimulating hormone levels. Thyroid function and mean SUV with increments of 1 were analyzed, and risk for thyroid dysfunction was calculated. Thyroid dysfunction detection ability was examined using a machine learning method (LightGBM, Microsoft) with age, sex, height, weight, CT value, volume, and mean SUV as explanatory variables.

RESULTS

Mean SUV was significantly higher in females with hypothyroidism. Almost 98.9% of participants in the normal group had mean SUV < 2 and 93.8% participants with mean SUV < 2 had normal thyroid function. The hypothyroidism group had more cases with mean SUV ≥ 2. The relative risk of having abnormal thyroid function was 4.6 with mean SUV ≥ 2. The sensitivity and specificity for detecting thyroid dysfunction using LightGBM (Microsoft) were 14.5 and 99%, respectively.

CONCLUSIONS

Mean SUV ≥ 2 was strongly associated with abnormal thyroid function in this large cohort, indicating that mean SUV with FDG-PET/CT can be used as a criterion for thyroid evaluation. Preliminarily, this study shows the potential utility of detecting thyroid dysfunction based on imaging findings.

PET/Computed Tomography in Thyroid Cancer

Primary thyroid cancers demonstrate distinct biological behaviors depending on their histologic characteristics. The ability to accumulate radioiodine by differentiated thyroid cancer cells is lost in primary aggressive, poorly differentiated and dedifferentiated tumor cells. PET imaging comes into play in these challenging situations where it can provide additive information to radioiodine scintigraphy and conventional imaging. This review focuses on the current guidelines and future prospects of PET imaging in thyroid cancers.

Graves' disease: A rare fate of Hashimoto's thyroiditis

Hashimoto's thyroiditis is a known autoimmune disorder that leads to chronic inflammation of the thyroid gland, with a gradual decline in function and eventual hypothyroidism. Conversion of Hashimoto's thyroiditis to Graves' disease is unusual clinically and has been occasionally reported in the literature. Awareness of such a rare phenomenon is important for the physicians evaluating patients with autoimmune thyroid disorders. Close investigation of the patient's symptoms and biochemical status is crucial in the implementation of appropriate treatment.

Detection of thyroiditis on PET/CT imaging: a systematic review

PURPOSE

The incidence and clinical significance of thyroiditis detected by molecular imaging methods is a clinical challenge that is not widely investigated in the literature. The aim of this systematic review was to analyze published data about the detection of thyroiditis on PET or PET/CT using different tracers.

METHODS

A comprehensive computer literature search of the Scopus, PubMed/MEDLINE, Embase, and Cochrane library databases was conducted up to November 2019 to find relevant papers on the detection of thyroiditis by PET/CT, the metabolic appearance, and the clinical significance.

RESULTS

Twenty-six articles were selected and retrieved in full-text version. From the analyses of these studies, the following main findings have been found. Diffuse thyroid uptake of PET tracers is a relatively frequent event, ranging from 0.4 to 46.2%, and it is commonly related to benign disease. Thyroiditis is the most frequent reason for diffuse increased thyroid ¹⁸F-FDG uptake. Cases of malignant disease with a pattern of diffuse ¹⁸F-FDG thyroid uptake are less frequent. Preliminary studies show a possible role of thyroiditis detected by ¹⁸F-FDG PET/CT in evaluating treatment response and as a prognostic marker in oncological patients treated with immunotherapy. However, further studies are needed.

CONCLUSIONS

Diffuse ¹⁸F-FDG thyroid uptake is a relatively rare event commonly due to benign diseases, among which thyroiditis is the most common. The rate of neoplastic disease with diffuse ¹⁸F-FDG thyroid uptake is very low. Diffuse ¹⁸F-FDG thyroid uptake requires further investigation and clinical evaluation for the correct diagnosis. Currently, cases of diffuse thyroid uptake with non-¹⁸F-FDG radiotracer are only anecdotal.