Disorders of the thyroid gland are diagnosed more often nowadays, probably because testing has become more of a routine procedure. Two recent publications have provided information that we summarize here, to help you understand the value of blood tests for thyroid malfunction.

Simply put, the thyroid may be over-active or under-active. Whichever the direction, the disorder may produce obvious symptoms, or it may be unnoticed (i.e. what physicians call 'subclinical').

In USA adults, thyroid malfunction occurs roughly as follows:

Under-activity (hypothyroidism)	2%
Subclinical under-activity	5 to 17%
Over-activity (hyperthyroidism)	0.2%
Subclinical over-activity	0.1 to 6.0%

The clinical signs mentioned earlier are shown in the following table:

Under-activity	Over-activity
Fatigue	Fatigue
Weight gain	Weight loss
Intolerance to cold	Intolerance to warmth
Dry skin, dry hair, hair loss	Clammy skin
Depression	Nervousness
Dementia	Sleeplessness, night terrors
Muscle cramps, pains	Muscle weakness
Edema	Shortness of breath
Slow heart rate	Palpitations, irregular heartbeat
Constipation	Frequent defecation

Some abnormal lab test results can indicate that your thyroid is not working properly. Such findings include raised cholesterol, low serum sodium level, raised serum calcium, raised liver enzymes, and anemia.

It must be emphasized that mild under-activity of the gland can readily progress to clinically obvious under-activity, with an increased risk of high cholesterol levels, irregular heartbeat and osteoporosis.

Three blood tests are now commonly used to help diagnose malfunction of the thyroid. They are:

• Thyrotropin (thyroid stimulating hormone, or TSH, the pituitary hormone that controls the activity of the thyroid gland). The normal levels range from 1 to 6 mU/L.
• Free thyroxine (FT4, a thyroid hormone produced by the gland). Normal levels are 1 to 3 ng/dL.
• Free triiodothyronine (FT 3, another hormone produced by the thyroid) normal levels are 75 to 200 ng/dL.

Using these three tests, six types of malfunction can be recognized:

Thyrotropin	FT4 or FT3	Commonest Cause(s)
raised (>10 mU/L)	normal	Subclinical under-activity (seen in 5-10% of all women)
raised	lowered	Clinically-manifest under-activity (hypothyroidism) with obvious symptoms (see above)
normal or raised	raised	An uncommon combination of results, seen with various non-thyroid conditions.
lowered (<0.1 mU/L)	normal	Seen in patients already taking thyroxine. Less commonly, subclinical over-activity (elderly).
lowered	raised	Over-active thyroid, due to Graves' disease, a nodular goiter (swollen thyroid), or a 'toxic' nodule. Sometimes called 'thyrotoxicosis'.
normal or lowered	lowered	Seen in non-thyroid illnesses; more rarely, if there is pituitary disease.

Because thyroid malfunction is quite common, and subclinical malfunction may require treatment to prevent other conditions developing, the American Thyroid Association recommends that thyrotropin is measured in adults every 5 years, starting at age 35. This simple screening test is every bit as cost-effectively as other widely accepted disease-detecting approaches, such as those for high blood pressure, high cholesterol, and breast cancer. If there are any symptoms or signs suggestive of thyroid malfunction, thyrotropin measurements should be done more frequently.

An abnormal thyrotropin result will probably necessitate additional tests, usually FT4 or FT3 in the first instance. More sophisticated tests may be ordered by your physician, or you may be advised to see a specialist. You should not be anxious about this, as thyroid diseases can almost always be managed quite satisfactorily.