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Letters to the Editor |
University of Pisa, Pisa, Italy
Stefano Mariotti
University of Cagliari, Cagliari, Italy
Lewis E. Braverman
University of Massachusetts, Worcester, Massachusetts
Aldo Pinchera
Institute of Endocrinology, University of Pisa, Pisa, Italy
We gratefully acknowledge the interest of Dr. Weissel for our recent paper on the clinical expression of myasthenia gravis (MG) associated with autoimmune thyroid disease (AITD) (1). His comments on the frequency of AITD in MG patients offer us the opportunity to expand this aspect, which was not specifically analyzed and discussed because this was not a primary object of our study. We offer the following reply.
As clearly indicated both in the title and the text of the paper (1), our study was not aimed to establish the actual frequency of the association between AITD and MG, but rather to evaluate the clinical features of MG associated with AITD, as opposed to MG without AITD. According to this design, an important proportion (38) of MG patients was selected from the bulk of patients referred to the Institute of Endocrinology for thyroid disease because of their associated MG. Epidemiological data can therefore be derived only from the 91 consecutive patients referred to the Institute of Neurology for MG evaluation and then submitted to systematic screening for thyroid disease. The overall prevalence of thyroid disease in these unselected MG patients was 42.8% (39/91), a value much lower than the 60% (77/129) calculated by Dr. Weissel by grouping together all our MG patients. Of the 39 unselected MG patients who were found to have associated thyroid disease, 13 (14.3%) had nonautoimmune thyroid diseases (mostly nontoxic nodular goiter). As stated in our manuscript, this finding is entirely in keeping with the expected prevalence of nontoxic goiter in an adult population from a mild to moderate iodine-deficient area such as Tuscany.
The prevalence of AITD in our MG series was 28.5% (26/91): 4.4% (4/91) with hyperthyroid Graves’ disease (GD), 3.3% (3/91) with euthyroid GD, 10.9% (10/91) with euthyroid Hashimoto’s thyroiditis (HT), and 9.9% (9/91) with hypothyroid HT or idiopathic myxedema.
We share the view of Dr. Weissel that literature data on the frequency of AITD in MG are unclear and often contradictory. Nevertheless, in the published reports quoted in our paper (2, 3, 4, 5, 6, 7, 8), as well as in the studies reviewed by Kiessling et al. (2), the prevalence of thyrotoxicosis in MG patients ranged from 3–10%, with frequencies of 5–6% in larger series. This is very much in line with our prevalence of 4.4% of hyperthyroid GD.
The prevalence of clinical hypothyroidism in MG patients calculated from the data reported in the above studies (2, 3, 4, 5, 6, 7, 8) ranged from 1.9–10.6%, again with a frequency of more than 5% when larger series were considered. Consistent with these studies are our data of a 9.9% prevalence, which includes both clinical and subclinical hypothyroidism.
The prevalence of euthyroid HT in our paper (10.9%) is similar to that (10.4%) calculated from the data of Scherbaum et al. (4), who took into account this type of AITD. In our series, euthyroid HT was defined by typical hypoechogenic pattern at thyroid ultrasound associated with circulating thyroid autoantibodies. In other studies the frequency of thyroid autoantibodies in euthyroid MG patients was assessed with no further investigation to identify autoimmune thyroiditis. It is of interest that thyroid autoantibodies associated with euthyroidism were found in 20 of the 104 (19.2%) patients with MG studied by Kiessling et al. (9), and that this group had a significantly higher TSH response to TRH compared with thyroid autoantibody-negative patients, suggesting an increased risk for hypothyroidism. By taking into account these considerations, it would be of interest to ascertain how many of the 16% euthyroid patients with thyroid autoantibodies found by Dr. Weissel in his series of 55 MG patients had some evidence of subclinical autoimmune thyroiditis.
Finally, our study (1) clearly showed that the prevalence of thyroid autoimmunity greatly varies with the severity of MG as assessed by the Osserman’s score. The overall prevalence of AITD calculated in our series by grouping together selected and unselected MG patients was 62.2% in those with ocular MG and 29.1% in patients with generalized MG (P < 0.002). Thus, any epidemiological study on the association between AITD and MG should consider separately these two clinical entities.
Footnotes
1 Address correspondence to:
Aldo Pinchera, MD, Institute of Endocrinology, University of Pisa, Via
Paradisa 2, Pisa, Italy 56124. 
Received August 11, 1997.
References
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