# Increased Pituitary Fluorine-18-Fluorodeoxyglucose Uptake in Patients with Differentiated Thyroid Cancer in Hypothyroidism versus under Recombinant Human Thyroid-Stimulating Hormone Stimulation

## TL;DR

This study shows that pituitary hypermetabolism seen in thyroid cancer patients on PET scans is often a normal response to low thyroid hormone levels, not a sign of disease.

## Contribution

The study demonstrates that pituitary hypermetabolism in thyroid cancer patients is a physiological response to hypothyroidism, not a pathological condition.

## Key findings

- Pituitary SUVmax and SUVratio were significantly higher in patients under thyroid hormone withdrawal compared to those under rhTSH stimulation.
- A positive correlation between serum TSH levels and pituitary SUVmax was observed only in the thyroid hormone withdrawal group.
- Pituitary hypermetabolism was more prevalent in the thyroid hormone withdrawal group (62.5%) than in the rhTSH group (23.5%).

## Abstract

The incidental pituitary hypermetabolism on 18F-FDG PET/CT should be further evaluated for discriminating between pathologic and physiologic uptake, but a recent study suggests that pituitary hypermetabolism is common in patients with differentiated thyroid carcinoma (DTC) undergoing thyroid hormone withdrawal (THW). The aim of this retrospective study was to compare pituitary metabolism in patients with DTC undergoing 18F-FDG PET/CT under THW versus recombinant human thyroid-stimulating hormone (rhTSH) stimulation. We confirmed a higher pituitary SUVmax and SUVratio with a higher prevalence of pituitary hypermetabolism in the THW group compared to the rhTSH group. A positive correlation between serum TSH levels and pituitary SUVmax was observed only in the THW group. The present findings support the hypothesis that pituitary hypermetabolism on 18F-FDG PET/CT in patients with DTC undergoing THW is a common physiological response to hypothyroidism. Awareness of this physiological hypermetabolism is important to avoid potential pitfalls in image interpretation that could trigger unnecessary investigations.

Background: 18F-FDG PET/CT is performed for the assessment of radioactive iodine non-avid disease in patients with DTC. In patients prepared by THW, increased pituitary uptake of 18F-FDG in the absence of pituitary disease may reflect the physiological activation of pituitary thyrotroph cells by hypothyroidism. This study aimed to compare pituitary 18F-FDG uptake in patients with DTC under THW vs. rhTSH stimulation. Methods: A total of 57 patients with DTC undergoing 18F-FDG PET/CT (40 under THW and 17 under rhTSH stimulation) were retrospectively analyzed. Pituitary metabolism was expressed as maximum standardized uptake value (SUVmax) and as SUVratio using the right cerebellum as reference. Results: Pituitary hypermetabolism (SUVmax ≥ 4.1) was present in more patients in the THW group compared to the rhTSH group (62.5% vs. 23.5%; p = 0.01). Pituitary metabolism was significantly higher in the THW group compared to the rhTSH group, as assessed by either SUVmax (mean ± SD: 4.61 ± 1.22, 95%CI: 4.22–5.00 vs. 3.34 ± 0.86, 95%CI: 2.9–3.8; p < 0.001) or SUVratio (0.52 ± 0.11, 95%CI: 0.49–0.56 vs. 0.42 ± 0.07, 95%CI: 0.38–0.46; p < 0.001). Serum TSH levels correlated positively with SUVmax (r = 0.41, p < 0.01) and SUVratio (r = 0.44, p < 0.01) in the THW group only. Conclusions: The present findings support the hypothesis that pituitary hypermetabolism on 18F-FDG PET/CT in patients with DTC undergoing THW is a common physiological response to hypothyroidism. Awareness of this physiological hypermetabolism is important to avoid potential pitfalls in image interpretation.