Duration of levothyroxine discontinuation and pre-radioiodine TSH levels in thyroid cancer.
Mintegui G G, Martínez Z Z, Basantes M M, Ferrando R R
Radioiodine treatment in differentiated thyroid carcinoma requires elevated TSH levels to optimize the uptake of [131I]NaI. Levothyroxine suspension is the usual strategy for inducing endogenous TSH elevation, although the optimal duration of this suspension remains controversial. To evaluate the effect of the duration of levothyroxine suspension on TSH levels achieved prior to radioiodine treatment. Retrospective cohort study that included 104 patients with differentiated thyroid carcinoma treated at a reference university center. Patients were classified according to the time of discontinuation of levothyroxine prior to radioiodine treatment: three weeks (group A, n = 21) and four weeks (group B, n = 83). TSH levels between groups and their distribution by categories (<30, 30-49, 50-69, 70-99 y ≥100 mIU/L) were analyzed. The proportion of patients who reached the recommended TSH threshold (≥30 mIU/L) was also evaluated. The mean age (SD) was 46.7 (10.3) years and 87 patients were women. The mean TSH was 70.9 mIU/L (95% CI: 54.5-87.2) in group A and 88.7 mIU/L (95% CI: 84.8-92.6) in group B. The proportion of patients who achieved TSH ≥ 30 mIU/L was significantly higher in group B (100% vs 76.2%; p < 0.001). The distribution of TSH levels by category differed significantly between both groups (p < 0.001). On the other hand, the proportion of patients with TSH ≥ 100 mIU/L did not show significant differences (60.2% vs 47.6%; p = 0.42). In multivariate analysis adjusted for age, sex, and radioiodine dose, levothyroxine discontinuation time was not independently associated with the likelihood of TSH ≥ 100 mIU/L. Three weeks of levothyroxine suspension allowed TSH levels ≥30 mIU/L to be reached in most patients, although a proportion did not reach the recommended threshold. Four weeks were associated with adequate TSH elevation in all patients in this cohort. TSH monitoring during preparation could allow for more individualized strategies that optimize thyroid stimulation, minimizing unnecessary exposure to hypothyroidism.