Tirosint Nicotine Interaction Profile: What Thyroid Patients Need to Know

Does nicotine interfere with Tirosint absorption?

The Tirosint capsule label names the drugs and foods that interfere with levothyroxine absorption: calcium carbonate and ferrous sulfate, bile acid sequestrants and ion-exchange resins, antacids, sucralfate and proton pump inhibitors, with a monitoring note for orlistat. Among foods it names soybean flour, cottonseed meal, walnuts, dietary fiber and grapefruit juice [1]. Nicotine does not appear among them [1].

Does smoking speed up levothyroxine the way it speeds up caffeine?

Cigarette smoke induces the liver enzyme CYP1A2, and polycyclic aromatic hydrocarbons in the smoke are believed responsible [5][2]. In one study, 13 smokers cleared caffeine with a half-life of 3.5 hours versus 6.0 hours in 13 non-smokers, and clearance was 155 versus 94 mL/kg/h [6]. In 12 heavy smokers who stopped abruptly, caffeine clearance fell 36.1%, from 2.47 to 1.53 mL/min/kg, with a de-induction half-life of 38.6 hours [7].

Reviews of smoking interactions name caffeine, clozapine, fluvoxamine, olanzapine, tacrine and theophylline as the main CYP1A2 substrates. They say dose adjustment or monitoring is clearly indicated for warfarin, olanzapine, clozapine and theophylline [2][3]. Levothyroxine is not among them [2].

Do smokers have different thyroid hormone levels?

A 1984 cross-sectional study of 441 women aged 48 to 53 from a general population (192 never smokers, 169 smokers and 80 former smokers) found higher serum T3 and lower reverse T3 in smokers than in never smokers, most pronounced at 20 or more cigarettes a day. Serum T4 did not differ, and the TSH difference was not statistically significant [4]. Those women were not selected for hypothyroidism, so the results do not describe how a Tirosint dose behaves.

Does nicotine itself induce liver enzymes?

In a rat study, a single subcutaneous nicotine dose induced lung CYP1A1 activity 4.4-fold at 6 hours and 14.7-fold at 12 hours, returning near baseline by 24 hours. Liver CYP1A2 protein and mRNA were not significantly changed [8]. The study did not measure thyroid hormone levels, and its relevance to humans is unknown. Nicotine's cardiovascular actions are better documented: peripheral vasoconstriction, faster heart rate and higher blood pressure [5].

Does quitting smoking change anything for Tirosint?

A review states that the hepatic enzyme effect of smoking is not related to the nicotine component of tobacco, so nicotine replacement will not alter it [3]. The drugs those reviews flag for dose changes after quitting are CYP1A2 substrates such as theophylline, clozapine, olanzapine and warfarin [3].