|Year : 2013 | Volume
| Issue : 2 | Page : 37-40
Pyramidal lobe on 99m technetium thyroid scan: Incidence, origin and association with thyroid function
Maseeh uz Zaman1, Nosheen Fatima2, Unaiza Zaman3, Zafar Sajjad1
1 Department of Radiology, Section of Nuclear Medicine, The Aga Khan University Hospital, Karachi, Pakistan
2 Department of Nuclear Medicine, Dr. Ziauddin Medical University, Karachi, Pakistan
3 Dow University of Health Sciences, Karachi, Pakistan
|Date of Web Publication||15-Jan-2014|
Maseeh uz Zaman
Department of Radiology, Section of Nuclear Medicine,The Aga Khan University Hospital, Karachi
Source of Support: None, Conflict of Interest: None
Purpose: The purpose of the study is to find out the incidence of pyramidal lobe (PL), its position in relation to thyroid and correlation with functioning status of thyroid gland. Materials and Methods : This was a retrospective study conducted in Nuclear Medicine Section, Department of Radiology, Aga Khan University Hospital, Karachi, Pakistan. We evaluated radionuclide thyroid scans performed with technetium-99m pertechnetate from March 2009 to May 2013. As per our departmental protocol, patients were used to swallow 100-150 cc water to wash away secreted esophageal activity and minimizing the possibility of false positive results. Patients with diagnosis of thyroiditis or history of any thyroid surgery were excluded. Presence and position of PL (arising from isthmus or either lobe of thyroid), presence or absence of cold or hot nodule (s), total thyroid uptake and thyroid function test performed within 4 weeks of thyroid scan were noted. Results: A total of 1116 thyroid scans qualified inclusion criteria of the study. PL was appreciable in 156 patients with overall incidence of 14% and an annual incidence of 3.5%/year (P < 0.05). Mean age of the population was 40 ± 12 years and female: Male was 113:43 (P < 0.05). Mean thyroid uptake was 14 ± 9% (P > 0.05). PL had origin from left thyroid lobe in 72 (46%), from the right thyroid lobe 58 (37%) and from the isthmus in 26 (17%) (P < 0.05). 135 (86.5%) patients were toxic (P < 0.05) while 15 (9.6%) euthyroid and 6 (3.8%) were hypothyroid (P > 0.05). Conclusion: We conclude that the incidence of PL on thyroid scanning was 14% with predominance for the female gender, left lobe origin and diffuse toxic goiters. An adequately performed thyroid scan would ensure completeness of thyroidectomies and avoid per-operative inconvenience to surgeons due to false positive scan results.
Keywords: Incidence, origin of pyramidal lobe, pyramidal lobe, thyroglossal duct, thyroid scan
|How to cite this article:|
Zaman Mu, Fatima N, Zaman U, Sajjad Z. Pyramidal lobe on 99m technetium thyroid scan: Incidence, origin and association with thyroid function. Saudi Surg J 2013;1:37-40
|How to cite this URL:|
Zaman Mu, Fatima N, Zaman U, Sajjad Z. Pyramidal lobe on 99m technetium thyroid scan: Incidence, origin and association with thyroid function. Saudi Surg J [serial online] 2013 [cited 2020 Aug 5];1:37-40. Available from: http://www.saudisurgj.org/text.asp?2013/1/2/37/125033
| Introduction|| |
Pyramidal lobe (PL) is often present and regarded as a normal variant. It was first described by Lalouette in 1749  and develops from the distal part of the thyroglossal duct.  The apex of PL is attached to the hyoid bone through a fibrous or muscular band called the levator glandulae thyroideae.  Due to significant variation in its prevalence in literature, it is regarded as an anomaly,  morphological variation  or normal component of the thyroid gland.  Precise localization of PL is important for a surgeon performing total thyroidectomy as a left in-situ PL may become a source of recurrence of disease. , Similarly in patients with well-differentiated thyroid cancers, removal of PL with thyroid gland is imperative to ensure successful ablation with radioiodine as residual PL may absorb appreciable amount of radioiodine and rendering ablation unsuccessful. 
The objective of this study was to find out the incidence of PL, its position in relation to thyroid and correlation with functioning status of thyroid gland.
| Materials and Methods|| |
This was a retrospective study conducted in Nuclear Medicine Section, Department of Radiology, Aga Khan University Hospital, Karachi, Pakistan. We evaluated radionuclide thyroid scans performed in our section from March 2009 to May 2013. Radionuclide thyroid scan was performed with 4.1-6.8 mCi (150-250 MBq) of technetium-99m pertechnetate ( 99m Tc-pertechnetate) injected intravenously. After 20 min, before starting imaging, patients were asked to take 100-150 cc of water to wash away secreted activity in the esophagus as it might be confused with PL (false positive results). Static images (anterior with and without hot marker over suprasternal notch and bilateral anterior oblique views) were acquired under a dual head digital gamma camera (Ecam Signature Series, Siemens, Germany) fitted with low energy high resolution collimators. Patients who showed PL on anterior images, were again asked to take 100-150 cc of plain water to wash out possible secreted activity in esophagus. Patients with diagnosis of thyroiditis (no or poor uptake on scan) or with history of any thyroid surgery were excluded from the study. All scans were reviewed and reported by a qualified nuclear physician (more than 10 years' experience). Presence and position of PL (arising from isthmus or either lobe of thyroid), presence or absence of cold or hot nodule(s), total thyroid uptake measured by computer software and serum thyroid stimulating hormone (TSH) with or without free-T4/total T3 (performed within 4 weeks of thyroid scan) were noted.
| Results|| |
During the study period, 1116 thyroid scans qualified the inclusion criteria of the study. Scans were indicated for evaluation of diffuse or nodular enlargement of thyroid, deranged thyroid function or symptoms suggestive of thyroid dysfunction (patients with a history of thyroid surgery or scan showing no uptake, i.e. thyroiditis were excluded from the study). PL was appreciable in 156 patients with overall incidence of 14% and an annual incidence of 3.5%/year (P < 0.05) [Table 1]. The mean age of the population was 40 ± 12 years (age range = 20-78 years) and 113/156 (72%) were female and 43/156 (28%) were male (P < 0.05). Mean tracer uptake over thyroid gland measured by computer algorithm was 14 ± 9% (P not significant). PL had origin from left thyroid lobe in 72 (46%), from right thyroid lobe 58 (37%) and from midline (isthmus) in 26 (17%) patients with significant P values for all locations [Figure 1]. According to thyroid function 135 (86.5%) patients were toxic, 15 (9.6%) euthyroid and 6 (3.8%) were hypothyroid (P value significant only for toxic cohort) [Table 1] and [Figure 2]. In patients with toxic profile, 93% had diffuse and 07% had nodular enlargement; in euthyroid group 80% had diffuse and 20% had nodular enlargement [Figure 3]. All hypothyroid patients had diffuse enlargement of thyroid with no evidence of cold nodule in any patient.
|Figure 1: Lateralization of pyramidal lobe on thyroid scintigraphy with reference to origin|
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|Figure 2: Incidence of pyramidal lobe with reference to thyroid function|
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|Figure 3: Incidental proportion of pyramidal lobe between diffuse and nodular goiter in correlation with thyroid function|
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| Discussion|| |
Overall incidence of PL in our study was low (14%) when compared with most of the reported studies. It has been reported to be in the range of 28  -55%  in cadaveric studies while in surgical series, reported incidence was 26  -65%.  Higher incidence in these series is due to direct visualization of the PL and did not have limitation associated with digital imaging. However, in one surgical study including 604 total thyroidectomies, intraoperative incidence of PL was 12% and only 50% of these were identified pre-operatively by ultrasound (U/S) and 99m Tc-pertechnetate thyroid scan (with false positive results of 4% and 8% respectively).  However, incidence in our study is comparable with a study published in 2007 with 10.7% incidence of PL visualization on 99m Tc-pertechnetate scan.  The plausible explanation of low incidence in our study could be limited resolution of the planar digital imaging, although we used to acquire anterior, right and left oblique images. We speculate that if single photon computerized emission tomography or pinhole imaging would have been done (which are not routinely performed for thyroid imaging in any busy nuclear medicine facility), we might have a higher incidence. Another explanation of this low incidence could be the pharmacokinetic of 99m Tc-pertechnetate as it is only taken up by functioning thyroid tissue and not organified like iodine ( 123 Iodine ideally or 131 I in case of non-availability of 123 I) with better chances detecting PL.  Our incidence is also lower than another study published from Islamabad, Pakistan with incidence of 41%.  The reason for this disparity among two studies from the same country could be lower sample size of previous study and propensity of subject having larger thyroid glands as studied subjects belonged to iodine deficient region of Pakistan. However, we cannot rule out the possibility of false positive results as a cause of higher incidence as Geraci et al.  reported incidence of 12% pre-operatively with a false positive rate of 8% with thyroid scanning. The seminal reason for false positive PL on thyroid scan is secreted salivary activity in the esophagus, which can be avoided by swallowing water before starting the thyroid scanning. In our department, we strictly follow this protocol which might have resulted in this low but presumably true positive incidence. However, to prove this notion a correlative study is strongly warranted.
Our study showed female predominance for the presence of PL and this is in concordance with various surgical and cadaveric series. ,, However in the literature, male preponderance has also been reported. ,
In this study, we have found that the majority of PL had significantly higher origin from left thyroid lobe followed by right lobe and isthmus. This correlates well with a many published surgical and cadaveric studies. ,,,,, However, study by Zivic et al.  on 100 consecutive thyroidectomies revealed the origin of PL from midline (49%) right (36%) and left (15%) side of the isthmus and no case of lobar origin. The reason for the left side predominance as reported in the majority of literature is not uncertain.
The mean thyroid uptake of 99m Tc-perthnetate in this study was significantly high (14 ± 9%) and this could be explained by the fact that >85% of studied subjects were hyperthyroid with a statistically significant P value. On the same note, we have observed significantly higher incidence of PL in patients with toxic goiter and importantly 93% of these hyperthyroid had diffuse goiter (i.e. Graves' disease). This predominance could be explained by generalized stimulation of TSH receptors by TSH receptor antibodies in Graves' disease which is not a case in toxic nodular goiters. 
This study draws attention of thyroid surgeons toward a lower but significant incidence of PL and importance of radionuclide thyroid scan for ascertaining its presence pre-operatively. PL can also be localized by radiological modality such as high resolution U/S, computed tomography (CT) or magnetic resonance imaging (MRI). High resolution U/S is a non-radiation based modality, but dependent upon operator expertise. CT and MRI are high resolution imaging modalities with limitations of high cost, intravenous contrast related issues and radiation exposure associated with CT. However, American Thyroid Association does recommend the use of U/S but no CT or MRI for patients undergoing thyroidectomies for well-differentiated thyroid cancers.  Localization and removal of PL during total or near total thyroidectomy is very important as left in-situ PL could become a site of tumor recurrence or may render adjuvant 131 I ablation unsuccessful. ,,,
We conclude that the incidence of PL on thyroid scanning was 14% with predominance for the female gender, left lobe origin and diffuse toxic goiters. An adequately performed thyroid scan would ensure completeness of thyroidectomies and avoid per-operative inconvenience to surgeons due to false positive scan results.
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[Figure 1], [Figure 2], [Figure 3]