Reviewed by MedNext Clinical Team
Levothyroxine (L-T4) is the standard treatment for hypothyroidism and one of the most prescribed medications globally. Despite decades of use, suboptimal prescribing — manifesting as under-treatment, over-treatment, or failure to account for absorption interactions — remains common. Optimising levothyroxine therapy requires attention to dosing initiation, monitoring, drug and food interactions, and patient-specific considerations [1].
Indications
The primary indication is hypothyroidism — whether autoimmune (Hashimoto's thyroiditis), post-surgical, post-radioiodine, or congenital. Levothyroxine is also used as suppressive therapy following thyroidectomy for differentiated thyroid cancer, and as part of combination therapy in some cases of refractory hypothyroid symptoms.
Initiating Therapy
In otherwise healthy adults with hypothyroidism, levothyroxine can be initiated at the estimated full replacement dose (approximately 1.6 micrograms/kg/day). In elderly patients, those with ischaemic heart disease, or those with severe or longstanding hypothyroidism, initiation at a low dose (25–50 micrograms daily) with gradual upward titration every 4–6 weeks is safer, as abrupt restoration of euthyroidism can precipitate angina or arrhythmia [1].
Administration and Food Interactions
Levothyroxine should be taken on an empty stomach, 30–60 minutes before breakfast, to maximise absorption. Several commonly used substances substantially reduce levothyroxine absorption and should be taken several hours apart: calcium supplements (at least 4 hours); iron supplements (at least 4 hours); antacids containing aluminium or magnesium (at least 2 hours); proton pump inhibitors (which reduce gastric acid and impair dissolution of levothyroxine tablets); coffee (reduces absorption by approximately 30% when taken simultaneously); and high-fibre foods taken in large amounts [1].
Patients should be counselled explicitly about these interactions, as many routinely take calcium or iron supplements without disclosing this, and apparent levothyroxine "resistance" often resolves on correcting absorption interference.
TSH Monitoring
TSH is the primary monitoring parameter for levothyroxine therapy. After initiating therapy or changing the dose, TSH should be re-checked after 6–8 weeks, as this is the time required for the pituitary-thyroid axis to reach a new equilibrium. Once stable, annual monitoring is sufficient for most patients, unless symptoms change or interacting medications are added [1].
The TSH target for most patients with primary hypothyroidism is the reference range of the assay (typically 0.4–4.0 mIU/L). In thyroid cancer suppression, lower TSH targets are set according to recurrence risk stratification. In elderly patients and those with cardiovascular disease, a target in the upper half of the reference range avoids the risks of subclinical hyperthyroidism.
Signs of Over- and Under-Treatment
Under-treatment (persistently elevated TSH) manifests as fatigue, cold intolerance, weight gain, constipation, dry skin, bradycardia, and cognitive slowing. Biochemically, elevated total cholesterol and anaemia may be present. Over-treatment (suppressed TSH) causes palpitations, tremor, heat intolerance, insomnia, diarrhoea, weight loss, and — with chronic exposure — increases in bone mineral density loss and atrial fibrillation risk. Both under- and over-treatment are common in practice and highlight the importance of regular monitoring.
References
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670-1751.