A case of Euthyroid Graves’ orbitopathy with negative TRAb antibody.

Original CArshiya Tabasum1, Ishrat Khan1, Peter Taylor1,2, Gautam Das1 and Onyebuchi E Okosieme1,2

A 66-year-old female presented with a 4-month history of double vision, excessive tearing, sticky feeling in the eyes, and orbital pain in all gaze directions.

Q  What is the importance of double vision in a case of suspected thyroid associated orbitopathy (TAO) ?

  • Double vision suggest involvement of extraocular muscles in case of TAO

She had no symptoms of thyroid dysfunction, did not smoke, and denied any personal or family history of thyroid disease.

She was clinically euthyroid and had no palpable goiter

Her visual acuity was 5/6 in both the eyes.

She had fullness of her eyelids on the right side with erythema below the right inferior orbital rim.

 

Q  Why is this finding important “She had fullness of her eyelids on the right side with erythema below the right inferior orbital rim”  ?

 

  • This finding suggests two things
    • The patient has eyelid swelling
    • The patient has erythema of the eyelid
  • Both of these are important points in the clinical activity score of TAO as published by EUGOGO.

 

She had right eye proptosis and diplopia on vertical gaze but with no lid lag or retraction.

Her intraocular pressures were normal and the optic discs were normal on fundoscopy.

 

Q  Which Extraocular muscle is commonly involved in TAO ?

 

  • Inferior rectus is most commonly involved followed by Medial rectus.

 

Q  What are the your differential diagnosis at this stage ?

 

  • This patient has unilateral proptosis with no lid retraction or lid lag.
  • Yet, Thyroid associated orbitopathy still remains my first differential diagnosis as it the most common cause of unilateral proptosis.
  • Apart from that Non specific orbital inflammatory syndrome (inflammatory pseudotumors) would be my second differential diagnosis
  • Orbital tumors , especially those arising from optic nerve (like optic nerve glioma) would be another differential diagnosis

 

At this point, a differential diagnosis of right inferior rectus mass and thyroid eye disease was considered.

 

Q  How will you investigate this case ?

 

  • First I would send thyroid function test with TSH Receptor antibody
  • Also I would consider doing a CT scan or the orbit.

 

 

Thyroid function test was normal: TSH 2.25 U/L (reference range 0.4–4.5), FT4 11.6pmol/L (reference range 11.0–24.0), and FT3 4.3pmol/L (reference range 2.67–7.03) (Table 1)

TRAbs and TPOAbs were negative and thyroid ultrasound scan showed no evidence of thyroid disease. TRAb measurement was performed using a commercial third-generation ELISA kit that detects both thyroid-stimulating (TSAbs) and -blocking antibodies (TBAbs) with manufacturer specificity and sensitivity of 100 and 95%, respectively, and positive cut-off of >0.4 U/L (RSR Laboratories, Cardiff, UK)

In the ELISA, serum TRAbs inhibit the binding of human biotin-labeled monoclonal antibody to immobilized recombinant TSH receptor on the ELISA plate. The amount of M22-biotin bound to the plate is then determined by the addition of streptavidin peroxidase and tetramethylbenzidine and the absorbance of the mixture is read at 450 nm using a plate reader

 

Q  What is the role of measurement of TSH receptor antibody ?

 

  • TSH receptor antibody would help in confirming the diagnosis of Graves’ disease and hence TAO
  • It will also help in assessing the severity of the disease, guide management decision and help in follow up of the patient

 

Q  How do TRAb antibodies predict the outcome of the patient with TAO ?

 

  • Higher TRAb levels at the time of diagnosis are associated with worse clinical course and poor outcomes.

 

Q  What are the subtypes of TSH receptor antibodies ?

 

  • Stimulating – TSI à Graves’ disease
  • Blocking – TBI or TBII – Atropic thyroiditis
  • Neutral

 

Q  What are the methods by which TSH receptor antibodies are tested ?

 

  • There are two major ways in which TSH Receptor antibodies are tested
  • The first is TBII assay (Receptor method) and the other is functional biological assay for TRAb.
  • TBII assay cannot differentiate between stimulating and blocking antibodies while the functional assay can differentiate between the two.

 

Q  Tell me something about TBII assay ?

 

  • TBII assay are based on the principle of TSH binding to the TSH receptor and the antibodies competing with this binding.
  • Naturally they donot distinguish between TSI and TBI (TBAbs)
  • The 2nd generation TBII assay uses radiolabelled TSH receptor for assay
  • The 3rd generation TBII assay uses recombinant TSHR and monoclonal antibody against TSI.

 

Q  Tell me about the biological assay for TRAb ?

 

  • The biological assay are based on the principle of cAMP generation on binding of ligand with TSH
  • TSI activates TSH receptor leading to generation of cAMP and TBI reduces the generation of cAMP
  • Current assays are based on Mc4-CHO cell lines
  • However interpretation of the results is sometimes difficult since the same patient may have blocking as well as stimulating antibodies.
  • Hence these are mainly used in research setting.

 

 

Q  What kind of assay is used in this case  ?

 

  • It is a TBII assay which is used
  • It would not distinguish between TSI and TBI

 

Q  What is your interpretation of the CT scan in this patient ?

 

  • The CT scan clearly shows inferior rectus thickening on both the sides.
  • Hence eventhough clinically the patient has mainly right sided proptosis, based on CT scan both the sides are involved.
  • Thickening of the EOM with sparing of the tendon insertion site is characteristic of TAO.
  • However, in this cases sparing of the tendon is not demonstrated as alternate cuts showing inferior rectus insertion are not provided.

fig1

A CT scan of the orbit showed bilateral asymmetrical enlargement of the inferior rectus, more marked on the right than on the left, and was highly suggestive of GO inflammation (Fig. 1). An MRI orbit (Fig. 2) also confirmed inflammation in keeping with GO (Fig. 2). There was no evidence of structural or other inflammatory lesions on imaging. Serum calcium and angiotensin-converting enzyme (ACE) levels were normal.

 

Q  So what is your diagnosis at end of the investigations ?

 

  • Since now there is CT evidence of bilateral involvement of the EOM, the diagnosis would most likely still be TAO.
  • Again absence of any other tumor or pathology would also favor the diagnosis of TAO, albeit euthyroid TAO.

 

Q  Since there is still no confirmed diagnosis of TAO, what would you consider doing next ?

 

  • Though this seems nothing else other than TAO based on clinical judgement, the patient being euthyroid and absence of significant elevation of TRAb antibody does create doubts about the diagnosis.
  • In option in such a case would be to perform FNAC or biopsy in such a scenario.

 

Q  What is the role of Functional imaging modalities in such cases of proptosis where diagnostic confusion exists ?

 

  • In a study done by García-Rojas et al , FDG- PET is useful for distinguishing EOM in TAO versus other conditions.
  • In their study they found that patients with TAO has higher SUVmax uptake of FDG in EOM compared to those having non thyroidal cause of EOM involvement.

 

 

Q  Assuming it is TAO, Based on the evidence provided, what is the Clinical activity score in this patient ?

 

  • The CAS would be 3 based on the evidence provided which would make it active ophthalmopathy.
  • According to the authors, the MRI is also suggestive of active inflammation.

 

Q  Describe the EUGOGO clinical activity score ?

  • EUGOGO Clinical activity score

fig2

Q  What is the severity of the TAO based on the evidence provided ?

 

  • Based on the ATA grading it would be Grade IV since there is involvement of EOM (NO SPECS classification)
  • It would be difficult to classify the patient based on the EUGOGO assessment of severity since the measurement of proptosis and severity of the soft tissue involvement is not provided.

 

She was treated symptomatically in the regional multidisciplinary Thyroid Eye Disease clinic with topical lubricants and fitted with a corrective prism. She later underwent a surgical recession of 4mm of the inferior rectus muscle with complete resolution of diplopia and headaches. A biopsy of the inferior rectus muscle taken at surgery showed a low-grade infiltrate with B and T lymphocytes. She had no further eye complaints and continued to be followed up with annual thyroid function tests.

 

Twenty-four months after initial presentation, she developed symptoms of anxiety, sweating, tremor, and palpitations. She had no goiter or eye signs. Her thyroid hormone levels were consistent with T3 toxicosis: TSH 0.04U/L, FT3 9.56mol/L, and FT4 15.5pmol/L. At this point, repeat TRAb and TPOAb measurements were positive: 7.6IU/L and 67.9IU/mL, respectively (Table 1). She was commenced on carbimazole initially and subsequently treated with a ‘block and replace’ regimen of carbimazole and levothyroxine. She responded satisfactorily with clinical and biochemical resolution and treatment was discontinued after 12 months. Her vision has since remained intact with no further disturbance of thyroid function.

 

Q  Describe the timing of diagnosis of TAO in relation with Graves’ hyperthyroidism ?

  • Timing with Graves’ hyperthyroidism
    • 20% – Precedes hyperthyroidism
    • 40%- diagnosed concurrently
    • 20%- Follows the diagnosis of hyperthyroidism
    • 20%- Occurs after Radio-active iodine therapy

 

Q  What percentage of people with TAO have hyperthyroidism, euthyroidism and hypothyroidism ?

  • With patients having TAO
    • 80%- Hyperthyroid
    • 10-20%- Euthyroid
    • 1-10%- Hypothyroid

 

Q  What are the features of Euthyroid TAO compared to hyperthyroid TAO  ?

 

  • Euthyroid TAO compared to hyperthyroid TAO are:
  • Milder
    • Less severe EOM
    • Unilateral more often
    • Lesser soft tissue involvement
    • Absence of lid lag and Upper eyelid retraction.
    • Lower degree of TRAb antibody

 

Q  What is the explanation of the absence of TRAb antibody in this case ?

 

  • Absence of TRAb antibody may be due to low titres of this antibody in case of Euthyroid graves orbitopathy.
  • The tests use for TRAb antibody donot have 100% sensitivity.
  • Antibodies against IGF-1 receptor have also been implicated in pathogenesis of TAO.

Learning points

  1. TRAb antibodies are an important test for diagnosis of TAO especially in cases of Euthyroid TAO or in cases where diagnostic confusion exists. However, in some cases of TAO, TRAb antibodies may be negative or in low titres to be detected at the time of early presentation.
  2. CT scan of the orbits is an important diagnostic tool in case of proptosis where diagnosis of TAO is not certain.
  3. FNAC, Biopsy, Surgical resection and FDG PET are other modalities used for differential diagnosis of proptosis.
  4. Onset of TAO may preceed the onset of hyperthyrodisim in patient with Graves’ disease in about 10-20% cases.
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