Introduction

Thyroid disorders are diagnosed more frequently nowadays in older persons, possibly because testing thyroid function is becoming a more routine procedure. Recently, a British specialist has reviewed our knowledge about the most usual tests and how to interpret them, and the American Thyroid Association has issued recommendations on the need for screening. These publications are summarized here.

Thyroid dysfunction

The prevalence of thyroid dysfunction in US adults is quite high:

Hypothyroidism	2%
Mild hypothyroidism	5-17%
Hyperthyroidism	0.2%
Mild hyperthyroidism	0.1-6.0%

Mild hypothyroidism, which is subclinical, is revealed by a raised serum thyrotropin (thyroid-stimulating hormone, or TSH) with a normal serum free thyroxine (FT4) concentration. Mild (subclinical) hyperthyroidism is a serum thyrotropin below 0.1 mU/L with normal free thyroxine and triiodothyronine levels.

The signs of clinically manifest (or overt) thyroid dysfunction are well known:

Hypothyroidism	Hyperthyroidism
Fatigue	Fatigue
Weight gain	Weight loss
Intolerance to cold	Intolerance to warmth
Dry skin, dry hair, hair loss	Clammy skin
Depression	Nervousness
Dementia	Insomnia, night terrors
Muscle cramps, myalgia	Muscle weakness
Edema	Dyspnea
Bradycardia	Palpitations, tachycardia, arrhythmias
Constipation	Frequent defecation

Risk factors for thyroid dysfunction include: the presence of a goiter, diabetes, vitiligo, pernicious anemia, prematurely gray hair (leukotrichia), and certain medications (e.g. amiodarone, iodine-containing agents), and a relevant family history.

Some lab test results suggest the possibility of an associated thyroid dysfunction:

Hypothyroidism	Hyperthyroidism
Hypercholesterolemia	Hypercalcemia
Hyponatremia	Elevated alkaline phosphatase
Anemia	Elevated serum transaminases
Elevated creatinine phosphokinase
Elevated lactate dehydrogenase
Hyperprolactinemia

Any of these clinical or lab findings indicate the need for thyroid function tests. Mild hypothyroidism can progress to overt hypothyroidism.

Common thyroid function tests

Serum thyrotropin (TSH) measurement using a sensitive method (i.e. with a limit of detection <0.1 mU/L) is normally the first, and sometimes the only, test done. Normal serum levels range from 1 to 6 mU/L.

Thyroid hormones - free thyroxine (FT4) should always be measured in those cases where secondary hypothyroidism due to pituitary or hypothalamic disease is suspected. Normal serum FT4 levels are 1 to 3 ng/dL. Free triiodothyronine (FT3) measurements - normal levels are 75 to 200 ng/dL - are valuable if there is clinical hyperthyroidism with normal FT4 levels.

Interpreting abnormal results

Using the three tests outlined above, 6 patterns of abnormal results can be recognized:

Thyrotropin	FT4 or FT3	Commonest Cause(s)
raised (>10 mU/L)	normal	Subclinical hypothyroidism (5-10% of all women), usually associated with antithyroid peroxidase (anti-TPO) antibodies.
raised	reduced	Primary hypothyroidism; due to chronic autoimmune thyroiditis (Hashimoto's), post-thyroidectomy, or post-radioiodine treatment. Endemic (iodine- deficiency) goiter in some areas.
normal/raised	raised	(uncommon) Acute psychiatric illness (early weeks), amiodarone therapy, or a genetic condition.
reduced (<0.1 mU/L)	normal	Taking thyroxine. Less commonly, subclinical primary hyperthyroidism (elderly). Often a multinodular goiter is present.
reduced	raised	Primary hyperthyroidism, caused by Graves' disease, a multinodular goiter, or a toxic nodule. Rarely, postpartum or postviral thyroiditis, or drugs (amiodarone, dopamine, glucocorticoids).
normal/reduced	reduced	Non-thyroidal illness; more rarely, pituitary disease.

Screening recommendations

Thyroid dysfunction is quite prevalent, and subclinical disorders often require treatment to lower the risk of complications. Thus mild hypothyroidism can be associated with reversible hypercholesterolemia, and, in some cases, cognitive impairment. Mild hyperthyroidism can lead to atrial fibrillation and lowered bone mineral density in older persons.

The American Thyroid Association point out that serum thyrotropin measurement in adults every 5 years is just as cost-effective as other widely accepted disease-detecting strategies, such as those for hypertension, breast cancer, and hypercholesterolemia. The cost-effectiveness is even more favorable in women and the elderly. The Association recommends, therefore, that serum thyrotropin measurements be done every 5 years, starting at age 35. The existence of symptoms or signs suggestive of possible thyroid dysfunction makes more frequent screening necessary.

An abnormal thyrotropin result will necessitate additional tests - probably FT4 or FT3 estimates, in the first instance. More sophisticated investigations (e.g. anti-TPO, radioiodine uptake) may be ordered by a thyroid-specializing endocrinologist.