Reduced thyroid function or hypothyroidism frequently develops unnoticed until more severe symptoms manifest themselves. It is estimated that 12% of the population in North America have abnormal thyroid function, and 60% of these individuals are unaware they have the condition. It’s a complex condition that can be caused by autoimmune disease, genetic predisposition and diet. Some of the early symptoms  are reduced energy, low mood, and weight gain – these symptoms can overlap with depression. In fact, the majority of people have both reduced thyroid function and depression symptoms, and about 20% of people with major depression have hypothyroidism. Hyperactive thyroid can also cause anxiety, heart palpitations and weight loss.

Reduced thyroid function is typically diagnosed through a blood test for the hormones T3, T4 and TSH. Elevated TSH levels above 2.5 milliunits per litre (mU/L) are outside the normal range, however individuals may not show any symptoms of the disease until levels exceed 10 mU/L. Hypothyroidism is treated with the synthetic hormone levothyroxine also known by the brand names Synthroid or Eltroxin.

Hypothyroidism with TSH levels between 4-10 typically does not require levothyroxine treatment, but for those with hypothyridism and major depression or bipolar disorder, several studies have shown that treatment with levothyroxine helps to restore energy and augment the mood improvement people experience with antidepressant treatment.  Pharmacogenetic testing, which predicts how a person will respond to medications based on their unique DNA, can help guide treatment with the mental health medications hyperthyroidism sufferers are often prescribed.

While treatment with levothyroxine can effectively reduce thyroid hormone levels to normal, it is not without its challenges.  Too much levothyroxine can lead to excessive weight loss, agitation and atrial fibrillation (AFib) – these symptoms occur when TSH levels drop down to 0.1 mU/L. Atrial fibrillation is not associated with synthetic thyroid hormone, and most commonly occurs in patients with a genetic predisposition to AFib who are hyperthyroid. Other important things to note with levothyroxine treatment are as follows:

• Levothyroxine is a poorly absorbed drug with a slow hormonal feedback loop – it takes a few weeks or even months to achieve a noticeable response.
• The drug dose must be individually adjusted according to body weight and TSH levels.
• Both thyroid function and levothyroxine absorption are affected by Vitamin D and iodine levels.
• Levothyroxine must be taken on an empty stomach, and individuals should not eat for several hours after taking the drug.
• Skipping doses or taking the drug at different times of day can lead to a longer time to improve thyroid function and depression.
• Both thyroid function and levothyroxine absorption are affected by Vitamin D and iodine levels.

As a precaution, it may be prudent to check Vitamin D blood levels while testing for T3, T4, and TSH. Vitamin D deficiency is also common in people with a dysfunctional thyroid. Restoring Vitamin D levels will help to improve both your mood and alleviate hypothyroid symptoms. Vitamin D deficiency contributes to tiredness, low mood, brittle hair and nails. Vitamin D supplements can improve depression symptoms and the health of your skin, hair and nails.

Excessive iodine blood concentrations can cause goitrous hypothyroidism. This condition is characterized by an enlarged thyroid gland and reduced thyroid hormone production. Often, endocrinologists will recommend an iodine-free diet to treat a goiter, which may help some people with autoimmune thyroid disease and postpartum thyroiditis to reduce thyroid symptoms. Consult with your endocrinologist to determine whether iodine-free diet could help you to improve thyroid function and depression symptoms.

Unlike many other drugs, Levothyroxine is not metabolized in the liver; therefore, it is not affected by variations in the liver enzymes. There is no way to predict a person’s response to levothyroxine or an optimal dose beforehand. Yet, most individuals with low thyroid function and depression require treatment with antidepressants, stimulants or pain killers to improve mood symptoms and function.

Pharmacogenetic tests such as Pillcheck can provide valuable insights on your response to over 50 antidepressants, stimulants, pain killers and many other medications that you may be prescribed over your lifetime. Pillcheck can assist your physician in selecting appropriate treatment options and drug doses to minimize your risks of side effects.

Summary

• Reduced thyroid function frequently leads to depression, but this often goes undiagnosed
• People with low thyroid function and depression with even subclinical hypothyroidism can benefit from treatment with a synthetic thyroid hormone called levothyroxine
• Reducing TSH levels to 2 mU/L is recommended to improve hyperthyroidism and depression symptoms
• Dietary adjustments might help to normalize Vitamin D and iodine levels which also affect thyroid function
• People with reduced thyroid function and depression may benefit from pharmacogenetic testing if they need additional treatment with antidepressants, stimulants, gastrointestinal drugs and painkillers

References:

Mirella P. Hage and Sami T. Azar The Link between Thyroid Function and Depression J Thyroid Res. 2012; 2012: 590648.

Wilmar M Wiersinga, M.D. Adult Hypothyroidism

Zhao, T., Chen, B.M., Zhao, X.M. et al. Subclinical hypothyroidism and depression: a meta-analysis. Transl Psychiatry 8, 239 (2018). https://doi.org/10.1038/s41398-018-0283-7

Cohen, B. M., Sommer, B. R., & Vuckovic, A. (2018). Antidepressant-Resistant Depression in Patients With Comorbid Subclinical Hypothyroidism or High-Normal TSH Levels. American Journal of Psychiatry, 175(7), 598-604.

Dr. Tammas Kelly MD (Author) The Art and Science of Thyroid Supplementation for the Treatment of Bipolar Depression Paperback – Feb 13 2018