Savvy Psychopharmacology

Effects of psychotropic medications on thyroid function

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Ms. L, age 53, presents to an inpatient psychiatric unit with depression, difficulty concentrating, fatigue, cognitive blunting, loss of appetite, increased alcohol intake, and recent suicidal ideation. Her symptoms began 3 months ago and gradually worsened. Her medical and psychiatric history is significant for hypertension, fibromyalgia, and chronic pain (back and neck), major depressive disorder (MDD; recurrent, severe), and generalized anxiety disorder (GAD). Ms. L’s current medication regimen includes lisinopril, 40 mg daily; fluoxetine, 60 mg daily; mirtazapine, 30 mg at bedtime; gabapentin, 300 mg twice daily; alprazolam, 0.5 mg twice daily as needed for anxiety; and oral docusate, 100 mg twice daily as needed. Her blood pressure is 124/85 mm Hg, heart rate is 66 beats per minute, and an electrocardiogram is normal. Laboratory workup reveals a potassium level of 4.4 mEq/L, blood urea nitrogen level of 20 mg/dL, serum creatinine level of 0.8 mg/dL, estimated creatinine clearance of 89.6 mL/min, free triiodothyronine (T3) levels of 2.7 pg/mL, thyroid-stimulating hormone (TSH) level of 7.68 mIU/L, free thyroxine (T4) level of 1.3 ng/dL, and blood ethanol level <10 mg/dL. In addition to the symptoms Ms. L initially described, a review of systems reveals word-finding difficulty, cold intolerance, constipation, hair loss, brittle nails, and dry skin.

To target Ms. L’s MDD, GAD, fibromyalgia, and chronic pain, fluoxetine, 60 mg daily is cross titrated beginning on Day 1 to duloxetine, 60 mg twice daily, over 4 days. Mirtazapine is decreased on Day 3 to 7.5 mg at bedtime to target Ms. L’s sleep and appetite. Due to the presence of several symptoms associated with hypothyroidism and a slightly elevated TSH level, on Day 6 we initiate adjunctive levothyroxine, 50 mcg daily each morning to target symptomatic subclinical hypothyroidism, and to potentially augment the other medications prescribed to address Ms. L’s MDD.

Thyroid hormone function is a complex physiological process controlled through the hypothalamic-pituitary-thyroid (HPT) axis. Psychotropic medications can impact thyroid hormone function and contribute to aberrations in thyroid physiology.1 Because patients with mental illness may require multiple psychotropic medications, it is imperative to understand the potential effects of these agents.

Antidepressants can induce hypothyroidism along multiple points of hormonal synthesis and iodine utilization. Tricyclic antidepressants have been implicated in the development of drug-iodide complexes, thus reducing biologically active iodine.2 Tricyclic antidepressants also can bind thyroid peroxidase, an enzyme necessary in the production of T4 and T3, altering hormonal production, resulting in a hypothyroid state.1 Non-tricyclic antidepressants (ie, selective serotonin reuptake inhibitors [SSRIs] and non-SSRIs [including serotonin-norepinephrine reuptake inhibitors and mirtazapine]) have also been implicated in thyroid dysfunction. Selective serotonin reuptake inhibitors have the propensity to induce hypothyroidism through inhibition of thyroid hormones T4 and T3.1,3 This inhibition is not always seen with concurrent reductions in TSH levels. Conversely, non-SSRIs can influence thyroid hormone levels with great variation, leading to thyroid hormone levels that are increased, decreased, or unchanged.1 Patients with a history of thyroid dysfunction should receive close thyroid function monitoring, especially while taking antidepressants.

Antipsychotics have a proclivity to induce hypothyroidism by means similar to antidepressants via hormonal manipulation and immunogenicity. Phenothiazines impact thyroid function through hormonal activation and degradation, and induction of autoimmunity.1 Autoimmunity may develop by means of antibody production or antigen immunization through the major histocompatibility complex.2 Other first-generation antipsychotics (FGAs) (eg, haloperidol and loxapine) are known to antagonize dopamine receptors in the tuberoinfundibular pathway, resulting in increased prolactin levels. Hyperprolactinemia may result in increased TSH levels through HPT axis activation.1 Additionally, FGAs can induce an immunogenic effect through production of antithyroid antibodies.1 Similar to FGAs, second-generation antipsychotics (SGAs) can increase TSH levels through hyper­prolactinemia. Further research focused on SGAs is needed to determine how profound this effect may be.

The Table1 outlines considerations for modifying psychotropic therapy based on the presence of concurrent thyroid dysfunction. Thyroid function should be routinely assessed in patients treated with antipsychotics.

Considerations for modifying psychotropic therapy based on the presence of thyroid dysfunction

Mood stabilizers are capable of altering thyroid function and inducing a hypothyroid state. Lithium has been implicated in both hypothyroidism and hyperthyroidism due to its inhibition of hormonal secretion, and toxicity to thyroid cells with chronic use, respectively.1,4 Hypothyroidism can develop shortly after initiating lithium; women tend to have a greater predilection for thyroid dysfunction than men.1 Carbamazepine (CBZ) can reduce thyroid hormone levels without having a direct effect on TSH or thyroid dysfunction.1 As with lithium, women tend to be more susceptible to this effect. Valproic acid (VPA) has been shown to either increase, decrease, or have no impact on thyroid hormone levels, with little effect on TSH.1 When VPA is given in combination with CBZ, significant reductions in thyroid levels with a concurrent increase in TSH can occur.1 In patients with preexisting thyroid dysfunction, the combination of VPA and CBZ should be used with caution.

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