Prolonged TSH Suppression After Antithyroid Therapy of Graves

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Question

Please give me your opinion for another patient: A 33 years old female with history of Hyperthyroidism since 2008. She had taken Neomercazole 5mg to10mg until she became pregnant in 2008. During her pregnancy she was prescribed PTU 100mg daily and she is continuing same medications for first 6months during pregnancy and later stopped in last 3months. After delivery: As her TSH was supressed: She started to use Neomercazole 5mg OD since Decemeber 2009 June 2010: TSH:0.02, Free T4:11.95 pmol/L (9.13-23.8). Total T3:0.76ng/Ml (0.79-1.49): During this period her Neomercazole was reduced to 2.5mg OD since her Free T4 level was low. AntiTPO:21.6 Iu/ml (N<18). AntiTG antibodies: 13.6 (N<100) Her Thyroid scan:in Sept 2007: No evidence of hot/cold nodules. Uptake 3.4% Ultrasound Thyroid :2007: Well circumscribed hypoechoic nodules varies in size from 2-5mm. Her recent Ultrasound Thyroid in 2010: Normal study. August 2010: TSH:0.07 (0.25-5), Free T4:19.5 (6.0-24.5), Free T3:5.9. She continued on Neomeraczole 2.5mg Once daily. Nov 2010: TSH:0.01 (0.25-5), Free T4: 22.9 (6.0-24.5) and Free T3:6.4 (2.0-7.8): She is Neomearcazole 2.5mg OD for last 4-5months. My query is still that her TSH is suppressed almost for year now even though her Free T4 is upper normal level. When Can I expect TSH to become normal.?

Dr Sreenivas Yadla, Bahrain

Response

While one can think about exotic ideas such as anti T4 or T3 antibodies, or interfering medications such as ASA or dilantin, the more logical idea is that your patient is intermittently thyrotoxic from forgetting her med, or that the “high normal” range is actually too high for her pituitary. I believe it would be most unusual for one episode of hyperthyroidism to cause TSH suppression for more than a year while she is continuously euthyroid, although I hesitate to say that it “could never happen”. Considering that she seems well and has no complaints, perhaps continuing the med for several more months is the easiest idea.

L De Groot, MD