Dosages vary according to the age groups and the individual condition of the person, body weight, and compliance to the medication and diet. Other predictors of the required dosage are sex, body mass index, deiodinase activity (SPINA-GD), and etiology of hypothyroidism. Annual or semiannual clinical evaluations and TSH monitoring are appropriate after dosing has been established. Levothyroxine is taken on an empty stomach about half an hour to an hour before meals. As such, thyroid replacement therapy is usually taken 30 minutes prior to eating in the morning. For patients with trouble taking levothyroxine in the morning, bedtime dosing is effective, as well. A study in 2015 showed greater efficacy of levothyroxine when taken at bedtime. Doses of levothyroxine that normalize serum TSH may not normalize abnormal levels of LDL cholesterol and total cholesterol.Poor compliance in taking the medicine is the most common cause of elevated TSH levels in people receiving appropriate doses of levothyroxine. 50 and older
For older people (over 50 years old) and people with known or suspected ischemic heart disease, levothyroxine therapy should not be initiated at the full replacement dose. Since thyroid hormone increases the hearts oxygen demand by increasing heart rate and contractility, starting at higher doses may cause an acute coronary syndrome or an abnormal heart rhythm. Pregnancy and breastfeeding
Hypothyroidism is common among pregnant women. A nationwide cohort study showed that 1.39% of all pregnant women in 2010 in Denmark received a prescription of levothyroxine during pregnancy.
A study of eight women suggested that coffee may interfere with the intestinal absorption of levothyroxine, though at a level less than eating bran. Certain other substances can cause adverse effects that may be severe. Combination of levothyroxine with ketamine may cause hypertension and tachycardia; and tricyclic and tetracyclic antidepressants increase its toxicity. Lithium, though, can cause hyperthyroidism (but most often hypothyroidism)  by affecting iodine metabolism of the thyroid itself, thus inhibiting synthetic levothyroxine, as well. Soy,  walnuts, fiber, calcium supplements, and iron supplements can also adversely affect absorption. A study found that cows milk reduces levothyroxine absorption.To minimize interactions, a manufacturer of levothyroxine recommends after taking it,  waiting 30 minutes to one hour before eating or drinking anything that is not water. They further recommend to take it in the morning on an empty stomach. Mechanism of action
Levothyroxine is a synthetic form of thyroxine (T4), an endogenous hormone secreted by the thyroid gland, which is converted to its active metabolite, L-triiodothyronine (T3). T4 and T3 bind to thyroid receptor proteins in the cell nucleus and cause metabolic effects through the control of DNA transcription and protein synthesis. Like its naturally secreted counterpart, levothyroxine is a chiral compound in the L-form. Pharmacokinetics
Absorption of orally administered levothyroxine from the gastrointestinal tract ranges from 40 to 80%, with the majority of the drug absorbed from the jejunum and upper ileum. Levothyroxine absorption is increased by fasting and decreased in certain malabsorption syndromes, by certain foods, and with age.