Thyroid testing has become more sophisticated and much easier to interpret as the state of the art has progressed. Whereas much of the old testing required a reference test in addition in order to correct for different individual body protein status, newer testing is much more sensitive and spot-on as a single reported value. Each condition has one or two tests that make the best diagnosis as a primary test and tell the best story for diagnosis and treatment.
Hypothyroidism is a low functioning thyroid state. It affects more than one percent of the general population and a much larger percentage in individuals over 60. Weakness, fatigue, anemia, electrolyte abnormalities and heart problems are some of the manifestations of a low thyroid. As the pituitary gland is the most sensitive in detecting a low thyroid and stimulating the thyroid gland appropriately, a serum TSH (thyroid stimulating hormone) is the most sensitive. An elevated level indicates a low thyroid state. Rare conditions such as complete pituitary failure can make an individual hypothyroid without a high TSH, but that is quite an unusual occurrence.
It makes logical sense that just drawing a serum thyroid level can be just as effective—in fact, a free thyroxine level (T4) does exactly that. Older T4 measurements used to need to be accompanied with a resin uptake test so the level could be corrected for protein binding abnormalities. Now, Free T4 is done by dialysis which makes the test accurate on its own. So why is the T4 a somewhat less sensitive test? The range of thyroid levels is quite a wide one amongst normal individuals. A level, for example, of total T4 can be from 5-11ng/dl, a normal free T4 from 0.8-1.8ug/dl. The TSH helps determine what the right level is for an individual. So, a free T4 of 0.8 for instance with a TSH of two is fine, but one with a TSH of 10 indicates a low thyroid state. Thus, the TSH is the most sensitive test for hypothyroidism.
Hyperthyroidism is a condition where too much thyroid is produced in the body. Sweating, weight loss, high heart rate, tremor, and heart disease may occur from this condition. Intuitively, a high T4 or free T4 level should seem to indicate a high thyroid state and that is the case. How about when the thyroid level is high normal and the diagnosis is suspected clinically? In those cases, the pituitary gland has been fed that signal and it attempts to turn off stimulation of the gland. As such, TSH is suppressed and almost negligible. Ultrasensitive TSH measurements are quite good in making this diagnosis, especially when the thyroid levels may still be in the upper normal ranges.
T3 is another thyroid test used on occasion to diagnose both hyper and hypothyroidism. T3 is the active form of thyroid hormone. Think of thyroid as a compound with four iodine molecules attached. When one is cleaved off it activates the compound and becomes T3. Even done by dialysis method it is a tougher test to interpret, since so many normal and abnormal body states influence it. A condition called T3 toxicosis can exist where T3 is inappropriately high relative to T4, and a completely suppressed TSH helps makes the diagnosis. Likewise, an opposite condition called “euthyroid sick” is another condition that is unique—it is when the body is critically ill and T4 (although maybe even normal) is not converted to T3 but the body’s demand for thyroid is lower (almost conserved). It is usually a problem where treating the primary condition in the body normalizes the thyroid status with no specific therapy needed.
Other thyroid tests that are noteworthy are antibody and immunoglobulin testing. Antithyroglobulin and antithyroperoxidase antibodies are usually elevated in Hashimoto’s thyroiditis, where the gland is enlarged and normal thyroid cells are replaced by non-functioning cells. Most cases of Hashimoto’s go on to hypothyroidism and need thyroid replacement. Antithyroglobulin antibodies and thyroid stimulating immunoglobulin may be elevated in high thyroid states such as Graves’ disease, where the body’s immune system causes unregulated overproduction of thyroid hormone that speeds up the metabolic process in the body continuously and over-taxes the heart. These cases need more expedient treatment with antithyroid drugs, radioactive iodine or surgery.
Underwriters use thyroid testing to assess the current state of thyroid disease in the body—in hypothyroidism whether adequate replacement of thyroid hormone is being given (and taken by the applicant), or if overproduction is being adequately treated so no marked and long-lasting cardiac complications develop. Most internists and general practitioners can handle uncomplicated thyroid disease, but endocrinologists are generally involved when more definitive and urgent treatments are required. In either case, adequate treatment of disease generally does not result in anything but a standard or preferred policy unless permanent damage to other body organs (most commonly the heart) has occurred.
