An underactive thyroid, also called hypothyroidism, can be caused by antidepressants. Although researchers don't understand how antidepressants suppress thyroid function, they've known for years that lithium can cause hypothyroidism and have recently implicated other antidepressants as well. Hypothyroidism can also occur on its own because of diet (for instance, inadequate minerals, vitamins, essential fatty acids, amino acids, or excess carbohydrates), an immune reaction, genetic susceptibility, or from radiation treatments for an overactive thyroid.
Regardless of the cause, hypothyroidism can lead to numerous problems, including but not limited to weight gain, excessive tiredness, decreased sexual function, and brain fog-all of which can mimic anti-depressant side effects. The condition can also cause depressed moods and render antidepressants ineffective. So you may think your medication isn't working when the problem is actually caused by an inadequate level of thyroid hormones.
Many people on antidepressants who have symptoms of hypothyroidism may not have the full-blown condition that can be diagnosed using a standard blood test, which measures the level of thyroid stimulating hormone (TSH). Instead, they have a milder form of the condition, called subclinical hypothyroidism. In the late 1980s, I became frustrated after the test results of many of my patients with multiple symptoms of hypothyroidism kept coming back normal. This can't be right, I thought. I decided to perform a thyroid-releasing hormone.
To summarize, if you have hypothyroidism, it can have adverse effects on your mental conditions, which can lead you to sink further into depression. There are three types of thyroid-releasing hormones - the thyroid-stimulating hormone (TSH), free thyroxine (T4), and free triiodothyronine (TJ). The standard thyroid hormone test, which measures just TSH, is not always sufficient, since TSH levels can be normal even when the thyroid is underactive.