Hypothyroidism — known as low thyroid — may cause depression. Hypothyroidism is a “condition in which the body does not get enough thyroid hormone for optimal brain and body functioning,” according to Gary S. Ross, M.D., in Depression & Your Thyroid: What You Need to Know.

Research has found a link between hypothyroidism and depression. For instance, there’s some evidence that people with depression tend to have higher rates of hypothyroidism than the general population (such as this study). A 2004 study found that 38 percent of older patients with hypothyroidism also reported symptoms of depression.

Unfortunately, hypothyroidism often goes undiagnosed. Some people simply aren’t tested for thyroid problems, while others are, but their lab tests come back “normal,” Dr. Ross notes.

The problem is that normal test results can be deceiving. Even individuals with subclinical hypothyroidism can have problems with mood and cognitive function. According to Ross, subclinical thyroid dysfunction shares many of its symptoms with clinical hypothyroidism.

Hypothyroidism and subclinical hypothyroidism appear to be more common in women with estimates of 2 percent and 7.5 percent, respectively. Anywhere from 5 to 15 percent of people with subclinical thyroid dysfunction may develop clinical hypothyroidism. (Ross believes these numbers are much higher.)

Research also suggests that people with subclinical hypothyroidism may be more prone to depression compared to people with normal thyroid functioning. (Here’s one study.)

Ross believes that all individuals with depression should be evaluated for thyroid problems. He writes:

There may be rare cases of depression that cannot benefit from thyroid treatment. Nevertheless, in every case of depression, it is optimal practice to test very thoroughly for thyroid dysfunction, much more thoroughly than is usually done in initial screening examinations. When the testing is thorough, then if anything is found in keeping with a low thyroid function, it is crucial to include some kind of thyroid treatment protocol in the overall treatment plan for maximum benefit to the patient.

Testing & Diagnosis

So what is thorough testing? In Depression & Your Thyroid, Ross lays out a step-by-step guide for testing and diagnosis. The first step is to figure out if you have any of the symptoms of low thyroid and to discuss this with your doctor. These are some of the signs of thyroid dysfunction. (You may experience only a few of these.)

  • Fatigue
  • Puffy face
  • Oversensitivity to cold
  • Difficulty concentrating or remembering things
  • Tingling or numbness in hands and legs
  • Hair loss
  • Dry skin
  • Weight gain
  • Difficulty breathing
  • Low blood pressure
  • Low body temperature
  • Slow pulse
  • Slow reflexes
  • Infertility or repeated miscarriages

Next, your doctor should conduct a physical examination, which will include checking your blood pressure, pulse, reflexes and thyroid gland. In people with low thyroid, blood pressure and pulse are low and reflexes are sluggish. Ross notes that during your physical exam, your thyroid gland tends to be normal.

Because people with low thyroid typically get cold easily and have a low temperature, Ross suggests keeping a record of your temperature every morning for five days. Keep a thermometer by your bed and check it before getting up or moving.

The first round of tests should include: Free T3; free T4; TSH (thyroid-stimulating hormone); antiperoxidase antibody and antithyroglobulin antibody. (Learn more here.)

The second round of tests includes a 24-hour urine sample for T3 and T4 hormones. (Sometimes the tests will include a TBII or thyroid-binding inhibitory immunoglobulin, but it’s not typically ordered.)

Doctors perform the third round of tests to absolutely confirm that a person has hypothyroidism. They may look at adrenal function, male and female hormones, virus and bacterial infections, intestinal parasites, molds, food sensitivities, minerals, toxic metals, liver, coagulation, antioxidants, amino acids and organic acids. Whether you have any of these tests will depend on your symptoms and the previous tests.

Some of these tests are more accurate than others; and all tests have their limitations. This is why it’s important to pay particular attention to your day-to-day symptoms. Ross writes:

No blood test can completely and accurately reveal how much of the thyroid hormones is actually reaching the cells, entering the cells, and successfully turning on energy-producing biochemistry of the cells. This is why it is vital to develop an awareness of the subtleties of your physical, mental and emotional symptoms. Then, you put your whole picture together with all of the information gleaned from your tests to determine whether a clinical trial of thyroid medication is warranted.

Again, even if your results come back “normal,” you still might have thyroid problems, and a comprehensive evaluation is necessary. Author and patient advocate Mary Shomon notes the importance of never accepting that your results are “normal” in this piece. She writes:

I hear from many thyroid patients who say “My thyroid tests were ‘normal’ but I still think I have a thyroid problem.” And my first question is, what was normal according to your doctor? As a thyroid patient who wants to feel well, you’re going to have to accept — and I know this can be frustrating– that you will have to become more knowledgeable, assertive and empowered when it comes to your health. And one of the most important steps is no longer relying on the phone call from the doctor’s office saying “Your thyroid tests came back normal.” Or the “Blood Test Results Summary” form letter in the mail that has “Thyroid, Urinalysis, Cholesterol, etc.” with little check marks indicating “OK” next to them. You need to know the actual numbers — actually, you need to have a hard copy of the actual lab results, and keep a file of them* — and you need to know what those numbers mean. Many people are not aware that for 10 years, physicians have not even been able to agree as to what is considered “normal” for the thyroid stimulating hormone – TSH – test. And beyond that, there are other issues, including normal TSH but abnormal T4/T3 — these are the actual thyroid hormones in the bloodstream — or normal TSH/T4/T3 but elevated antibodies — that can diagnose thyroid conditions.

Further Reading

This New York Times piece provides information on mental health and thyroid problems.