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Dear Doctor: My mother and father have hypothyroidism, as do my two younger sisters. Now I've been experiencing symptoms that would indicate thyroid problems. My doctor checked my T3 and TSH levels, but did not check T4, and said I did not need medication. Shouldn't she also have checked my T4?

Dear Reader: Your question suggests that you've done some homework. Good job! As you know, hypothyroidism is the term used when the thyroid gland doesn't produce enough thyroid hormone. The condition is quite common, occurring in about 4.6 percent of the population. Further, it's five to eight times higher in women than men.

The most common cause of hypothyroidism is Hashimoto's thyroiditis, a slow autoimmune attack of the thyroid gland that leads to decreased production of thyroid hormone. Genetic susceptibility can indeed play a role in an autoimmune attack of the thyroid, meaning having one family member with hypothyroidism increases an individual's risk of hypothyroidism. Because you have multiple family members with hypothyroidism, I can understand your concern about developing this disease.

Measuring the level of TSH, thyroid-stimulating hormone, is a very good test for hypothyroidism. TSH, which is produced by the pituitary gland, stimulates the thyroid to produced thyroid hormone. Alternatively, in a feedback mechanism, the thyroid hormones T3 and T4 suppress secretion of TSH. So, if your thyroid hormone levels are high, your TSH level should be low, and if your thyroid hormone levels are low, your TSH level should be high.

In other words -- because of the feedback mechanism -- when you have hypothyroidism, your TSH should be higher than normal. Because the feedback mechanism is slow to change the level of TSH, the TSH level gives an indication of your overall thyroid status over many weeks. One caveat with TSH is that it increases as you get older, meaning that people over the age of 80 can have higher TSH levels, but not be considered hypothyroid. Because your TSH is normal, I would think that hypothyroidism is of low likelihood.

But let's take a closer look at T4 and T3. The thyroid produces these hormones in response to TSH; 80 percent of the hormones produced by the thyroid is T4, and only 20 percent is T3. The latter is a much more active thyroid hormone, with three to five times the effect of T4 and a much shorter half-life. That means its levels vary more than T4 levels from one day to the next. Also, T3 drops substantially with illness. It's true that T4 can be a more precise test for thyroid hormone status, but in your case I don't think there is additional value in checking T4 because both your TSH and T3 levels are normal.

That said, while your thyroid function on these tests are currently within range, your doctor should continue to monitor them because your family history puts you at increased risk of hypothyroidism. Be aware, however, that severe illness can affect the levels of TSH, T4 and T3, so thyroid function assessments are not accurate under those circumstances. At those times, checking a reverse T3 may be helpful.

Finally, keep in mind that the symptoms of hypothyroidism (fatigue, constipation, cold intolerance, weight gain and dry skin) can be caused by many other conditions. It's certainly smart to have your thyroid checked when you have these symptoms, but if your thyroid is normal, you should probably consider a broader evaluation.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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