Is lithium therapy suitable for a patient with pre-existing Hashimoto's (autoimmune thyroiditis) thyroiditis who is currently taking levothyroxine (thyroid hormone replacement medication)?

Lithium Can Be Used in Hashimoto's Disease with Appropriate Thyroid Monitoring

Lithium therapy is not contraindicated in patients with pre-existing Hashimoto's thyroiditis who are taking levothyroxine, but requires vigilant thyroid function monitoring and potential levothyroxine dose adjustments. 1

FDA-Approved Guidance on Pre-Existing Thyroid Disease

• The FDA label explicitly states that "previously existing underlying thyroid disorders do not necessarily constitute a contraindication to lithium treatment" 1
• When hypothyroidism exists, careful monitoring of thyroid function during lithium stabilization and maintenance allows for correction of changing thyroid parameters 1
• Supplemental thyroid treatment (levothyroxine) may be used when hypothyroidism occurs or worsens during lithium therapy 1

Mechanism of Lithium's Thyroid Effects

• Lithium decreases thyroid hormone synthesis and release at the cellular level 2
• It reduces peripheral conversion of T4 to T3 by inhibiting type I 5'-deiodinase enzyme activity 2
• Lithium augments B lymphocyte activity and reduces the ratio of suppressor to cytotoxic T cells, potentially worsening autoimmune thyroiditis in susceptible individuals 2

Expected Thyroid Complications on Lithium

• Hypothyroidism occurs in approximately 20% of patients on long-term lithium therapy 3
• Goiter develops in up to 40% of patients receiving lithium 3
• Lithium increases thyroid autoimmunity if autoantibodies are present before therapy initiation 3
• The patient's pre-existing Hashimoto's disease places her at higher risk for worsening hypothyroidism on lithium 2

Monitoring Protocol for This Patient

• Measure TSH, free T4, and thyroid antibodies (anti-TPO, anti-thyroglobulin) at baseline before starting lithium 2
• Recheck thyroid function tests every 6-8 weeks during lithium stabilization 4
• Once stable on lithium, monitor TSH and free T4 annually at minimum 2
• More frequent monitoring (every 3-6 months) is recommended for middle-aged females with pre-existing thyroid autoantibodies 2
• Assess thyroid size clinically and consider thyroid ultrasonography at baseline and annually 2

Levothyroxine Dose Adjustment Strategy

• Increase levothyroxine dose by 12.5-25 mcg if TSH rises above 4.5 mIU/L during lithium therapy 4
• Target TSH should remain within the reference range of 0.5-4.5 mIU/L with normal free T4 levels 4
• Recheck TSH and free T4 6-8 weeks after any levothyroxine dose adjustment 4
• Treatment with levothyroxine is effective for lithium-induced hypothyroidism, and lithium therapy should not be stopped 3

Critical Safety Considerations

• Never discontinue lithium due to thyroid dysfunction—adjust levothyroxine instead 3
• Patients must discontinue lithium and contact their physician if signs of lithium toxicity develop (diarrhea, vomiting, tremor, ataxia, drowsiness, muscular weakness) 1
• Maintain adequate fluid intake (2500-3000 mL daily) and normal salt intake during lithium therapy to prevent sodium depletion 1
• The patient's existing levothyroxine therapy provides a therapeutic advantage, as thyroid hormone replacement can be readily adjusted 1

Rare but Serious Autoimmune Complications

• Lithium can induce Hashimoto's encephalopathy in rare cases, characterized by elevated antithyroid antibodies, elevated CSF protein, and neurological symptoms 5
• This complication responds dramatically to intravenous methylprednisolone pulse therapy 5
• Monitor for new neurological symptoms (confusion, weakness, lethargy, tremulousness) that cannot be explained by lithium toxicity alone 5

Absorption Considerations with Hashimoto's Disease

• Patients with Hashimoto's disease frequently have comorbid gastrointestinal disorders (gastroparesis, SIBO, gastritis) that impair levothyroxine absorption 6
• If TSH remains elevated despite appropriate levothyroxine dose increases, consider switching to levothyroxine sodium oral solution (Tirosint-SOL), which contains only levothyroxine, water, and glycerol for improved absorption 6
• Standard levothyroxine tablets may be inefficiently absorbed in patients with Hashimoto's disease and GI comorbidities 6

Potential Therapeutic Benefit of Levothyroxine in Hashimoto's

• Prophylactic levothyroxine treatment in euthyroid Hashimoto's patients reduces both TPO antibodies and B lymphocytes after one year 7
• The patient's existing levothyroxine therapy may actually provide some protective effect against progression of autoimmune thyroiditis 7
• This represents an additional rationale for maintaining adequate levothyroxine dosing during lithium therapy 7

Common Pitfalls to Avoid

• Do not assume thyroid dysfunction on lithium requires discontinuation of lithium—adjust levothyroxine dose instead 3
• Do not overlook the need for baseline thyroid function testing before initiating lithium in patients with known thyroid disease 2
• Avoid infrequent monitoring—annual TSH checks are insufficient during the first year of lithium therapy in patients with pre-existing Hashimoto's disease 2
• Do not attribute all neurological symptoms to lithium toxicity without considering rare autoimmune complications like Hashimoto's encephalopathy 5