Adequate levothyroxine doses for the treatment of hypothyroidism newly discovered during pregnancy.

BACKGROUND: Recent guidelines recommend thyrotropin (TSH) target levels of ≤2.5 mIU/L for the first trimester and ≤3 mIU/L for the subsequent trimesters. Euthyroidism should be attained as soon as possible, but there are no precise indications about the initial levothyrorine (LT4) dose. The aim of our study was to determine the appropriate LT4 doses in order to normalize TSH levels in patients with newly discovered subclinical hypothyroidism (SCH) during pregnancy, and to correlate them with basal TSH levels. The adequate LT4 doses for women with SCH were also compared to those required in pregnant women with overt hypothyroidism (OH).

METHODS: Seventy-seven patients with newly diagnosed hypothyroidism during pregnancy were retrospectively analyzed. Patients were assigned to group 1 (n = 64) with SCH or group 2 (n = 13) with OH. SCH patients were subdivided into two groups: group 1a serum TSH >2.5 (1st trimester) or >3 (2nd or 3rd trimester) to 4.2 mIU/L; and group 1b TSH level > 4.21-10 mIU/L. All patients were treated with LT4 as soon as hypothyroidism was diagnosed. The dose that allowed a TSH of ≤2.5 mIU/L to be reached in the first trimester or one that allowed a TSH of ≤3 mIU/L to be reached during the second and third trimesters was considered the appropriate one.

RESULTS: A significant difference (p < 0.0001) in the appropriate LT4 dose (mean ± SD, μg/kg/day) was observed between group 1 and group 2: 1.31 ± 0.36 versus 2.33 ± 0.59. Patients in group 1a required a significantly lower LT4 dose (p < 0.014) than group1b: 1.20 ± 0.39 versus 1.42 ± 0.31 μg/kg/day. In 57 of the 64 (89.06%) women with SCH and in 10/13 (76.92%) women with OH, the appropriate LT4 dose coincided with the initial dose. Only 11% and 23% respectively required additional adjustments. Once the appropriate dose of LT4 was prescribed, the time at which euthyroidism (mean ± SD, weeks) was confirmed was similar in patients with SCH (6.06 ± 3.3) and OH (5.3 ± 1.8). There were no miscarriages or premature deliveries.

CONCLUSIONS: When hypothyroidism is newly discovered during pregnancy, we suggest initiating the treatment with the following LT4 doses: 1.20 μg/kg/day for SCH with TSH ≤ 4.2  mIU/L, 1.42 μg/kg/day with TSH > 4.2-10, and 2.33 μg/kg/day for OH. By taking this approach, patients will promptly attain the euthyroid state avoiding additional increments and, probably, obstetric risks.