Is It Your Thyroid? How Hypothyroidism Is Diagnosed

Fatigue, weight gain, feeling cold, thinning hair and brain fog are some of the most common reasons people start to wonder about their thyroid. It's a reasonable question, since an underactive thyroid can cause every one of those symptoms. The difficulty is that many other conditions can cause them too.

This post covers how hypothyroidism is diagnosed, the conditions that commonly look like it, its most common cause (Hashimoto's thyroiditis) and what to do when results fall into a gray zone. We'll also touch briefly on the opposite problem, an overactive thyroid.

This is part 2 of a three-part thyroid series. Earlier: Part 1: How Your Thyroid Works, and What Your Lab Results Can Tell You.

Why Symptoms Alone Aren't Enough

When clinicians have tried to identify hypothyroidism based only on symptoms, they haven't been able to reliably tell people with low thyroid function apart from people with normal thyroid function. On top of that, people who have classic hypothyroid symptoms but normal lab results generally don't feel better when they're given thyroid hormone.

For that reason, diagnosis starts with lab testing. Symptoms still matter a great deal, though, especially once treatment begins and the dose needs fine-tuning.

Conditions That Look Like Hypothyroidism

Several common conditions produce symptoms that closely resemble an underactive thyroid:

• Perimenopause and menopause. This is probably the most common source of confusion, since the symptoms overlap heavily and the timing often coincides. Checking FSH along with TSH can help tell them apart.
• Iron deficiency and anemia.
• Poor or insufficient sleep.
• Under-eating. Prolonged fasting, or eating too little for your activity level, lowers T3 as the body conserves energy. Endurance athletes and people in weight-sensitive sports are especially prone to this; in that case, the underlying issue is the energy shortfall rather than the thyroid itself.
• Heavy alcohol use.
• Medications. Some drugs, such as lithium and amiodarone, can affect thyroid function directly. Oral estrogen changes total T4 without affecting thyroid function, and many other medications cause fatigue, constipation or weight changes that can be mistaken for a thyroid problem.

Low morning body temperature deserves a brief mention as well, since it's often promoted as a home test. People with hypothyroidism do tend to run a little cooler, but a lower temperature on its own doesn't mean the thyroid is the cause.

How Hypothyroidism Is Diagnosed

Diagnosis relies mainly on two tests: TSH and free T4.

In overt primary hypothyroidism, TSH is clearly elevated and free T4 falls below the normal range. When antibody results and symptoms fit that picture, the diagnosis is straightforward.

A less common form, called central or secondary hypothyroidism, occurs when the pituitary gland or hypothalamus doesn't send an adequate signal to the thyroid. In that case, TSH may be low or normal even though free T4 is low. This form is rare, and it still requires a low free T4; if free T4 is normal, the thyroid is producing hormone as it should.

Hashimoto's: The Most Common Cause

In the United States, most cases of hypothyroidism are caused by Hashimoto's thyroiditis, an autoimmune condition in which the immune system gradually attacks the thyroid. Over the years, the gland can lose much of its ability to produce hormone.

This process usually unfolds slowly. For a long time, TPO antibodies may be positive while the thyroid still keeps up with the body's needs; free T4 stays normal, and TSH may drift upward. Positive antibodies with a normal free T4 don't necessarily call for treatment, but they do call for closer monitoring, particularly when TSH is rising or there's a family history of thyroid disease.

Not every case of hypothyroidism is autoimmune, however. Thyroid surgery, radioactive iodine treatment, certain medications and conditions present from birth can also lead to low thyroid function.

Antibodies and Pregnancy

Positive TPO antibodies are especially relevant for anyone planning a pregnancy. Women with these antibodies have higher rates of miscarriage and preterm birth, even when their thyroid function is normal, likely because autoimmune conditions tend to cluster together.

Giving thyroid hormone to antibody-positive women with normal thyroid function hasn't been shown to improve live birth rates, so the main benefit of knowing is closer monitoring. Thyroid hormone needs rise during pregnancy, and a gland under autoimmune attack may have trouble keeping up. If you're planning a pregnancy, testing TPO antibodies beforehand is a good idea.

It's also helpful to know that autoimmune thyroid disease is several times more common in women than in men.

The Gray Zone: Subclinical Hypothyroidism

Sometimes TSH comes back above the normal range while free T4 remains normal, often without any symptoms at all. This is called subclinical hypothyroidism, and it's one of the most common thyroid questions we see.

The first step is usually patience. A large share of mildly elevated TSH results return to normal on repeat testing, so rechecking labs in a couple of months is a sensible starting point. From there, a family history, TPO antibody testing and sometimes a thyroid ultrasound (which can show changes typical of early Hashimoto's) help fill in the picture.

Treatment becomes more reasonable when antibodies are positive, the ultrasound shows those changes, there's a strong family history or TSH continues to climb. A markedly elevated TSH is usually treated even without obvious symptoms, and many people notice afterward that they feel better than they'd realized was possible.

Age plays a role as well. The normal range for TSH tends to rise after about age 50, so a TSH of 7 or 8 may be perfectly sufficient for someone in their 80s. For an older adult who feels well and has a mildly elevated TSH, starting thyroid hormone usually isn't necessary.

Two Common Pitfalls

Thyroid care can go wrong in two opposite directions, and some people unfortunately experience both.

On one side, true thyroid disease can be missed when a single result lands inside the normal range. For example, someone with positive antibodies, ultrasound changes and a family history of Hashimoto's shouldn't be dismissed just because their TSH came back at 4.2.

On the other side, some people are labeled hypothyroid even though their TSH and free T4 are normal, often based on a borderline free T3 or an elevated reverse T3. Treatment in that situation carries risks of its own, since too much thyroid hormone can cause heart rhythm problems such as atrial fibrillation, as well as bone loss, anxiety and poor sleep.

Our approach is to make decisions based on the full picture, to keep monitoring when results are borderline and to treat when the evidence supports it.

When the Thyroid Is Overactive

Hyperthyroidism, or an overactive thyroid, is much less common than hypothyroidism, and it's rarely subtle. The two main causes are Graves' disease, in which antibodies stimulate the thyroid to overproduce hormone, and thyroid nodules that make hormone on their own.

People with hyperthyroidism often notice a pounding or racing heart, heat intolerance, sweating, tremor, anxiety, trouble sleeping and weight loss despite a normal appetite. Lab tests typically show a very low TSH with elevated free T4 and free T3.

There's also a milder, subclinical form in which TSH is low but hormone levels are still normal; the lower the TSH, the more likely it is to progress. Because an overactive thyroid can affect both heart rhythm and bone density, this is another good reason to include TSH in routine blood work. Treatment options include antithyroid medication, surgery and radioactive iodine, and those decisions are best made with an endocrinologist.

In the final post of this series, we'll cover the treatment options for hypothyroidism, how to take thyroid medication and what to watch for over the long term.

Asymmetric Health's thyroid care page explains the service available in Lacey and statewide by telehealth. To talk through your symptoms and existing lab reports, contact the clinic.