RESULTS
Of 557 patient charts studied, 100 patients were included; the mean (SD) age at nodule diagnosis was 62.4 (11.1) years, and the mean (SD) initial thyrotropin level at nodule diagnosis was 1.51 (0.87) μIU/mL. The mean (SD) most recent thyrotropin level was 1.60 (1.03) μIU/mL after a mean duration of 5.7 (2.5) years postnodule diagnosis (Table 2).
Six patients (6%; 95% CI, 2.5%-12.7%) who had a normal thyrotropin level at nodule diagnosis developed an abnormal thyrotropin level in a mean (SD) of 6.9 (3.1) years. These 6 patients had a mean age at nodule diagnosis of 69.2 (11.4) years. Five of the 6 were male, and all were White patients. One patient’s thyrotropin level rose from an initial thyrotropin of 3.38 μIU/mL at nodule diagnosis to a high of 7.76 μIU/mL after 8.5 years. This patient was diagnosed with subclinical hypothyroidism and did not require treatment.
Five patients’ thyrotropin levels dropped below normal in a mean 7 years, with levels ranging from 0.25 to 0.52 μIU/mL. Of these patients, 2 became symptomatic from the nodules, experiencing dysphagia or hoarseness, with 1 diagnosed with hyperthyroidism. This patient was treated with methimazole and radioactive iodine ablation 9 years after diagnosis. The other 3 patients who developed low thyrotropin had no nodule symptoms or treatment. Ninety-four patients maintained thyrotropin values in the normal range for a mean (SD) of 5.7 (2.5) years and had a mean (SD) age at nodule diagnosis of 61.9 (11.0) years.
Both thyrotropin groups were compared. For categorical variables, there were no significant differences for sex (P = .99) or race (P = .55). For continuous variables, there were no significant differences in age at diagnosis (P = .12), initial thyrotropin (P = .24), most recent thyrotropin (P = .98), or time from diagnosis to most recent thyrotropin level (P = .23) (Table 3).
Of note, the VADHS changed the type of blood sample used to generate thyrotropin values from serum to plasma and had 3 different normal reference ranges during the 2010 to 2016 period studied. The thyrotropin values fell into 4 categories: serum sample with normal range 0.4 to 5.5 μIU/mL, serum sample with normal range 0.4 to 4.0 μIU/mL, plasma sample with normal range 0.4 to 4.0 μIU/mL, and plasma sample with normal range 0.6 to 4.8 μIU/mL. There were no significant differences between the abnormal and normal most recent thyrotropin groups in sample type for initial or most recent thyrotropin (P = .44 and P = .99, respectively) or in normal range for initial or most recent thyrotropin level (P = .99 and P = .09, respectively).
DISCUSSION
We found no statistically significant change in blood thyrotropin levels over time among patients with thyroid nodules with no history of medical conditions or medications known to affect thyroid hormone levels. Six of 100 patients developed abnormal thyrotropin, but only 2 eventually were treated for thyroid dysfunction: 1 for hypothyroidism and 1 for hyperthyroidism. The other 4 patients who did not receive treatment developed low thyrotropin but had no official diagnosis of hyperthyroidism in their health records, seemingly due to lack of multiple, consistently low thyrotropin values or due to lack of follow-up. Based on these data, monitoring thyrotropin over time may not be necessary in patients without any medical history known to affect thyroid function. The results provide support for the original hypothesis.