Endocrine Abstracts (2010) 22 P853

Marked sensitivity of pituitary feedback to levothyroxine in asymptomatic primary autoimmune hypothyroidism

Raluca-Alexandra Trifanescu1,2 & Mihail Coculescu1,2

1‘Carol Davila’ University of Medicine and Pharmacy, Bucharest, Romania; 2‘C.I.Parhon’ Institute of Endocrinology, Bucharest, Romania.

Background: Few cases of hypersensitivity of thyroid–pituitary feedback in hypothyroid children were reported.

Aim: To present a family with asymptomatic, severe biochemical primary autoimmune hypothyroidism who showed a marked sensitivity of pituitary feedback to levothyroxine treatment.

Methods: TSH was measured by 3rd generation immunoassay kit.

Case report: R.G, woman, aged 22, presented for asymptomatic goiter. Family history consisted in autoimmune hypothyroidism in proband’s mother (TSH=375 mIU/l, FT4=0.46 ng/dl, FT3=1.17 pg/ml, TPO antibodies=392 IU/ml) and euthyroid autoimmune thyroid disease (TSH=0.7 mIU/l, FT4=1.35 ng/dl, TPO antibodies=30.4 IU/ml), vitiligo and recurrent urticaria in her sister. Physical exam revealed a medium goiter, with no signs of hypothyroidism. Mental development and stature were normal. Biochemical data showed severe primary autoimmune hypothyroidism (TSH=121 mIU/l, FT4=0.4 ng/dl, antithyroglobulin antibodies (Ab)=771.9 IU/ml). TSH receptor Ab, TPO Ab were negative. Treatment with levothyroxine 50 μg/day for 6 weeks determined subclinical thyrotoxicosis (TSH=0.04 mIU/l, FT4=1.5 ng/dl). Subsequent decrease in levothyroxine dose to 25 μg/day rapidly led in 2 weeks to TSH increase up to 18 mIU/l and decrease in thyroid hormones (FT4=0.86 ng/dl, FT3=0.85 pg/ml). One month after levothyroxine’s withdrawal, severe biochemical hypothyroidism occurred once again, without any clinical signs (TSH=173.8 mIU/l, FT4<0.4 ng/dl, FT3=0.74 pg/ml).

There is a hypersensitivity to levothyroxine’s effect both centrally (suppressed TSH at low levothyroxine doses, administered for short time) and peripherally (absence of myxedema signs despite of very low FT4 levels for many months).

Conclusion: It is tempting to suggest that thyroid–pituitary feedback hypersensitivity could be an additional cause of hypothyroidism along with thyroid autoimmunity in this case. Genetics of thyroid hormones’ receptors and of deiodinases could also be involved in this family.

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