Paradoxical Decrease in TSH Level during Thyroxine Withdrawal for Radioiodine Therapy for Well-Differentiated Thyroid Cancer—A Case Report

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Introduction: Elevated TSH is required for effective treatment of well differentiated thyroid cancer with radioiodine. This can be achieved by the use of recombinant TSH or thyroxine withdrawal. Recombinant TSH is expensive and not available in Ghana.

Methods: A 52-year-old male patient was diagnosed with follicular thyroid cancer from the histopathological evaluation of the sample from a pathological fracture of his right humerus. He received external beam radiation to the right femur and subsequently underwent a total thyroidectomy but was started on 150 micrograms levothyroxine before being referred for radioiodine treatment. He was asked to stop the levothyroxine for 6 to 8 weeks before radioiodine treatment. Patients reported with a serum levothyroxine taken after 5 weeks of withdrawal to determine whether the TSH had arisen to acceptable levels. A clinical decision is made to continue withdrawal of the levothyroxine for another 4 weeks before radioiodine was ordered. The aim of this study is to discuss why thyroid withdrawal may result in the desirable serum TSH needed for radioiodine treatment in thyroid cancer.

Results: The serum TSH was 16 mIU/L after 5 weeks and dropped to 8 mIU/L after a further 4 weeks of levothyroxine withdrawal. The radioiodine was ordered and administered to patient 2 weeks after the last serum TSH and another serum TSH done before radiopharmaceutical therapy showed a further decrease in serum TSH to 5 mIU/mL. Patient was treated with 7,400 MBq of sodium [¹³¹I] iodide. A posttherapeutic scan showed widespread metastasis involving predominantly the skeletal system.

Discussion: In well-differentiated thyroid cancer, especially the follicular thyroid cancer, the cancer cells retain the ability of the thyrocyte to trap and produce thyroid hormone. In our patient the initial withdrawal after thyroidectomy led to the elevation of TSH. This elevated TSH stimulated the metastatic thyroid cells to produce enough thyroid hormone to start producing negative feedback on the pituitary and hypothalamus resulting in the subsequent decrease in TSH. The postscan showed iodine uptake, but this may have been underestimated. A year after initially treatment patient had another treatment and this time his serum TSH levels were 45 mIU/mL suggesting the previous treatment had reduced the burden of metastatic disease which was confirmed on the posttherapeutic scan for the second treatment.

Conclusion: Patients with widespread metastatic well-differentiated thyroid cancer may not have a sufficient elevation of serum TSH for radioiodine treatment after levothyroxine withdrawal or produce a paradoxical decrease in serum TSH on prolonged levothyroxine withdrawal.

Publication History

Article published online:
25 May 2023