DEAR DOCTOR K:
My doctor thinks I may have hypothyroidism. How will he make the diagnosis?
Hypothyroidism is the medical term for an underactive thyroid. Some conditions are hard to diagnose, but fortunately hypothyroidism is not one of them.
Your thyroid is a small gland in your neck that makes the thyroid hormones, called T3 and T4. These hormones leave the gland and travel in the blood to every cell in your body. Thyroid hormones influence the rate at which every cell, tissue and organ in your body functions.
Hypothyroidism occurs when your thyroid gland doesn’t produce enough thyroid hormones. Your body slows down, creating symptoms such as fatigue, depression, weight gain, feeling cold for no good reason, constipation and dry skin.
Your thyroid gland is controlled by another gland: the pituitary gland, which is in the brain. It sends a chemical message to your thyroid, telling it how much hormone to make. The chemical message is called thyroid-stimulating hormone (TSH).
Your brain is constantly sensing whether there is an adequate level of thyroid hormones circulating in your blood. If not, the pituitary makes more TSH. Higher levels of TSH prompt the thyroid to produce more thyroid hormones. Low TSH levels signal the thyroid to slow down production.
To diagnose hypothyroidism, your doctor will perform a physical examination and some blood tests:
- Physical exam. Your doctor will assess the size of your thyroid by feeling around your neck. He or she will check for physical signs of hypothyroidism, such as coarse hair or hair loss, dry or yellowish skin, and pale or puffy appearance. Your weight, cholesterol levels and blood pressure will also be checked.
- TSH test. This test is the best way to determine if you have thyroid disease. (I’ve put a table showing the normal, low and high values for the TSH and other thyroid blood tests below.) If your TSH level is high, you are hypothyroid. If TSH levels are below normal, you are hyperthyroid: Your thyroid gland is making too much thyroid hormone. (There are exceptions to this, but they are infrequent.)
- T4 and T3 tests. Once secreted by the thyroid, only a small amount of T4 is “free” and available for immediate use. In hypothyroidism, there’s not enough free T4 in the blood. There’s also not enough T3 in the blood.
Together, the TSH, free T4 and T3 tests can establish a diagnosis. They can also indicate how severe your problem is.
Thyroid blood test results
The ranges below apply to people who are not yet taking thyroid medications.
|Condition||TSH level*||Free T4 level||Free T3 level|
|Normal (euthyroid)||Within normal range of 0.45–4.12 mIU/L||Within normal range of 0.8–2.0 ng/dL||Within normal range**|
|Central (also known as secondary) hypothyroidism, indicating a pituitary disorder||Low||Low||Not useful|
|Central (also known as secondary) hyperthyroidism, indicating a rare pituitary abnormality producing TSH||High||High||High|
|Subclinical (mild) hypothyroidism||High||Within normal range||Not useful|
|Subclinical (mild) hyperthyroidism||Low||Within normal range||Within normal range|
|*An abnormal TSH level can indicate a range of problems depending on whether the pituitary gland is functioning normally.**The normal ranges in the bloodstream vary from lab to lab.Sources: Third U.S. National Health and Nutrition Survey; Subclinical Thyroid Disease: Scientific Review and Guidelines for Diagnosis and Management, 2004 (Consensus Panel Recommendation).|
Fortunately, treating hypothyroidism is easy. You take thyroid hormone in pill form, to replace the hormone your thyroid gland is not making enough of. The same thyroid blood tests used to diagnose hypothyroidism also are used to determine if you’re taking the right dose of thyroid medicines. These tests are repeated regularly, since a person’s dose can change over time.
Diagnosing hypothyroidism has become much simpler and more precise since I went to medical school.