Endocrine Abstracts

Introduction: Many studies have shown that cigarette smoking exerts multiple effects on the thyroid gland. Smoking seems to induce changes in thyroid function tests, like a left-shift in serum TSH level that is more apparent in iodine-deficient subjects and increase in thyroid hormones. We describe the effect of an intensive thyroxine intravenous treatment in severe hypothyroid patients and the results depending on their smoking status.

Material and methods: Our retrospective study includes 70 consecutive severe hypothyroid patients (mean TSH of 104.1 mUI/L (±34.6)) treated from 2004 to 2018 at our institution with intravenous levothyroxine with a mean dose of 433.3 micrograms in two days and then treated orally with a standard replacement dose. We describe their evolution at 1 and 4 weeks comparing TSH, free T4 and free T3 levels and days it took hormonal levels to normalize. Data were analyzed with the Statistical Package for Social Science (SPSS), version 20.0 (SPSS Inc., Chicago, IL).

Results: TSH levels were higher among non-smokers but its response to intravenous levothyroxine was faster and it took fewer days to normalize after treatment due to different free hormone levels despite a smaller oral replacement dose.

Smokers vs Non smokers:: Age 49.1 (+7.6) 54.6 (+8.3), P 0.04; Weight 72.9 (+11.8) 68.9 (+12.4), P 0.002; pre-treatment TSH 95.2 (+14.6) 110.01 (+21.3), P 0.04; TSH 2–3 days post-treatment 85.1 (+8.1) 61.3 (+9.7), P 0.001; TSH 10–30 days post- treatment 59.7 (+15.3) 20.2 (+8.4), P 0.001; TSH > 30 days post-treatment 11.6 (+4.6) 4.4 (+3.2), P 0.001; Pre-treatment T4L 0.3 (+0.1) 0.42 (+0.3), P>0.05; T4L 2–3 days post-treatment 0.4 (+0.1) 0.84 (+0.2), P>0.05; T4L 3–10 days post-treatment 0.6 (+0.2) 1.0 (+0.3), P 0.003; T4L 10–30 days post-treatment 1.0 (+0.3) 1.0 (+0.4), P 0.001; T4L >30 days post-treatment 1.1 (+0.2) 1.25 (+0.3) P 0.001; Pre-treatment T3L 2.6 (+0.3) 2.0 (+0.6), P 0.02; T3L 2–3 days post-treatment 2.9 (+0.4) 2.5 (+0.2), P 0.02; T3L 3–10 days post-treatment 2.9 (+0,6) 2.6 (+0.7), P 0.02; T3L 10–30 days post-treatment 2.8 (+0.4) 2.8 (+0.5), P 0.001; T3L >30 days post-treatment 2.7 (+0.6) 2.3 (+0.3), P 0.02; Oral T4 doses on discharge 130.7 (+23.2) 114.6 (+21.5), P 0.001; Days to normalize free T4 8.3 (+1.2) 7.6 (+1.3), P>0.05; Days to normalize free T3 7.4 (+0.6) 9.3 (+0.7), P 0.04; Days to normalize TSH 44.7 (+7.8) 28.5 (+5.4), P 0.001.

Conclusions: Response to intravenous levothyroxine treatment is different depending on the tobacco habit of patients, with a left-shift on TSH levels probably due to altered thyroid hormone metabolism.