What is Hypothyroidism?
Hypothyroidism is a condition in which the thyroid gland does not produce sufficient thyroid hormones to meet the body's metabolic demands. The thyroid, a butterfly-shaped gland located at the base of the neck, produces two primary hormones: thyroxine (T4) and triiodothyronine (T3). These hormones regulate virtually every metabolic process in the body, including heart rate, body temperature, energy expenditure, protein synthesis, and cholesterol metabolism.
The most common cause of hypothyroidism worldwide is iodine deficiency, while in iodine-sufficient countries, autoimmune thyroiditis (Hashimoto's disease) is the leading cause. Other causes include surgical removal of the thyroid (thyroidectomy), radioactive iodine therapy, certain medications (lithium, amiodarone), and pituitary disorders. Hypothyroidism affects approximately 5% of the adult population, with a significantly higher prevalence in women and older adults.
Symptoms develop gradually and may include fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, depression, bradycardia, and cognitive slowing. Severe untreated hypothyroidism can progress to myxedema coma, a life-threatening emergency characterized by hypothermia, altered consciousness, and multi-organ dysfunction.
Levothyroxine: Mechanism of Action
Levothyroxine (L-T4) is a synthetic form of thyroxine (T4), the predominant hormone produced by the thyroid gland. It is bioidentical to endogenous T4 and serves as the standard of care for thyroid hormone replacement therapy. Once absorbed, levothyroxine circulates in the blood bound primarily to thyroid-binding globulin (TBG), albumin, and transthyretin.
The physiologically active hormone is actually T3, not T4. Approximately 80% of circulating T3 is produced by peripheral conversion of T4 through the action of deiodinase enzymes in the liver, kidneys, and other tissues. This conversion mechanism allows the body to regulate T3 levels locally based on tissue-specific needs, which is one reason why T4 monotherapy is preferred over combination T4/T3 therapy for most patients.
Levothyroxine has a long half-life of approximately 6–7 days, which provides stable serum levels with once-daily dosing and allows for missed doses without dramatic fluctuations. After initiating therapy or adjusting the dose, it takes approximately 4–6 weeks for serum TSH to reach a new steady state, which is why TSH monitoring intervals are set at 6–8 weeks after each dose change.
Dosing Guidelines
Levothyroxine dosing varies considerably based on the clinical scenario. The following table summarizes the standard dosing approach for each indication:
| Indication | Patient Population | Starting Dose | Titration |
|---|---|---|---|
| Primary hypothyroidism | Young adult (<50, no cardiac disease) | 1.6 mcg/kg/day (full replacement) | Recheck TSH in 6–8 weeks |
| Primary hypothyroidism | Elderly (>50) or cardiac disease | 25–50 mcg/day | Increase by 12.5–25 mcg every 4–6 weeks |
| Subclinical hypothyroidism | All patients | 25–75 mcg/day | Adjust based on TSH response |
| TSH suppression | Post thyroid cancer | 2.0–2.5 mcg/kg/day | Target TSH <0.1 mU/L |
For young, otherwise healthy patients with primary hypothyroidism, full replacement dosing at 1.6 mcg/kg/day can be initiated immediately. This typically results in starting doses of 75–125 mcg/day depending on body weight. In elderly patients or those with coronary artery disease, a conservative approach starting at 25–50 mcg/day is essential because rapid thyroid hormone replacement can precipitate angina, arrhythmias, or myocardial infarction in susceptible individuals.
Obese patients may require higher absolute doses but lower weight-based doses, as lean body mass correlates more strongly with levothyroxine requirements than total body weight. Pregnancy increases levothyroxine requirements by 25–50% due to increased TBG levels and expanded plasma volume, and dose adjustments should begin as soon as pregnancy is confirmed.
TSH Targets
The goal of levothyroxine therapy is to normalize TSH and alleviate symptoms. Recommended TSH targets vary by clinical context:
- Most patients with hypothyroidism: TSH 1.0–2.0 mU/L (within the lower half of the reference range, typically 0.4–4.0 mU/L)
- Elderly patients (>70 years): TSH 4.0–6.0 mU/L may be acceptable, as overtreating can increase risks of atrial fibrillation and osteoporosis
- TSH suppression (high-risk thyroid cancer): TSH <0.1 mU/L
- TSH suppression (low-risk thyroid cancer): TSH 0.1–0.5 mU/L
- Pregnancy: TSH <2.5 mU/L in the first trimester, <3.0 mU/L in the second and third trimesters
Overtreatment (excessively suppressed TSH) carries significant risks including atrial fibrillation, osteoporosis, anxiety, and insomnia. The minimum effective dose needed to achieve the target TSH should always be used.
Drug Interactions
Levothyroxine has numerous clinically significant drug interactions that can reduce its absorption or alter its metabolism:
| Interacting Agent | Mechanism | Management |
|---|---|---|
| Calcium supplements | Binds T4 in GI tract, reducing absorption | Separate by 4 hours |
| Iron supplements | Binds T4 in GI tract, reducing absorption | Separate by 4 hours |
| Proton pump inhibitors | Reduced gastric acidity impairs T4 dissolution | May need higher dose; monitor TSH |
| Antacids (aluminum/magnesium) | Binds T4 in GI tract | Separate by 4 hours |
| Cholestyramine / colestipol | Binds T4 in GI tract | Separate by 4–6 hours |
| Estrogen / oral contraceptives | Increases TBG, reducing free T4 | May need dose increase; monitor TSH |
| Rifampin, phenytoin, carbamazepine | Increases hepatic T4 metabolism | May need dose increase; monitor TSH |
Administration Tips
Proper administration of levothyroxine is critical for consistent absorption and therapeutic efficacy:
- Take on an empty stomach: Levothyroxine should be taken 30–60 minutes before breakfast or at bedtime (at least 3 hours after the last meal). Food reduces absorption by 20–40%.
- Take with water only: Coffee, milk, and juice can all impair absorption. Take the tablet with a full glass of plain water.
- Consistency is key: Whether taken in the morning or at night, the most important factor is taking it the same way every day relative to meals and other medications.
- Avoid crushing: Swallow tablets whole. If unable to swallow, dissolve in a small amount of water immediately before taking.
- Separate from interfering medications: Wait at least 4 hours before taking calcium, iron, or antacids.
- Store properly: Keep at room temperature, away from light and moisture. Check expiration dates regularly.
Side Effects
When dosed correctly, levothyroxine has minimal side effects because it replaces a hormone the body normally produces. Most adverse effects result from overtreatment (iatrogenic hyperthyroidism):
- Cardiovascular: Tachycardia, palpitations, arrhythmias (especially atrial fibrillation), angina, increased blood pressure
- Neurological: Tremor, anxiety, irritability, insomnia, headache
- Gastrointestinal: Diarrhea, nausea, increased appetite
- Metabolic: Weight loss, heat intolerance, excessive sweating, menstrual irregularities
- Musculoskeletal: Muscle weakness, osteoporosis (with chronic overtreatment)
If side effects occur, the dose should be reduced. In elderly patients or those with cardiac disease, even modest overtreatment can have serious consequences. Allergic reactions to levothyroxine itself are extremely rare; most reported reactions are to inactive ingredients (dyes, fillers) in specific formulations.
Frequently Asked Questions
How long does it take for levothyroxine to work?
Patients typically begin to notice symptom improvement within 2–3 weeks, but it takes 4–6 weeks for TSH levels to stabilize after a dose change. Full clinical benefit may not be apparent for 2–3 months. TSH should be rechecked 6–8 weeks after any dose adjustment.
Can I take levothyroxine with other thyroid medications?
Some patients take combination T4/T3 therapy (levothyroxine plus liothyronine), though this remains controversial. The American Thyroid Association recommends T4 monotherapy as the standard of care, with combination therapy considered only for patients who remain symptomatic despite optimal TSH levels.
What happens if I miss a dose?
Because levothyroxine has a long half-life (6–7 days), missing a single dose has minimal clinical impact. Take the missed dose as soon as you remember, or take a double dose the next day. Do not take more than two days' worth at once. Consistent daily dosing is preferable for stable hormone levels.
Why are there so many tablet strengths?
Levothyroxine has a narrow therapeutic index, meaning small dose changes can produce significant clinical effects. The wide range of available strengths (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 mcg) allows for precise dose titration in increments as small as 12.5 mcg, minimizing the risk of under- or overtreatment.
Should the dose change during pregnancy?
Yes. Levothyroxine requirements typically increase by 25–50% during pregnancy due to increased thyroid-binding globulin levels and expanded blood volume. Women on levothyroxine should increase their dose by approximately 30% as soon as pregnancy is confirmed and have TSH checked every 4 weeks during the first half of pregnancy.
Is brand switching a concern?
Yes. Different manufacturers' levothyroxine products may have slightly different bioavailability. The ATA recommends maintaining the same formulation whenever possible. If a brand switch occurs, TSH should be rechecked in 6–8 weeks to ensure levels remain in target range. Some patients are particularly sensitive to formulation changes.