Tests Assessing the Effects of Thyroid Hormone on Body Tissues

Thyroid hormone regulates a variety of biochemical reactions in virtually all tissues. Thus, ideally, the adequacy of hormonal supply should be assessed by the tissue responses rather than by parameters of thyroid gland activity or serum hormone concentration which are several steps removed from the site of thyroid hormone action. Unfortunately, the tissue responses (metabolic indices) are nonspecific because they are altered by a variety of physiologic and pathologic mechanisms unrelated to thyroid hormone deprivation or excess. The following review of biochemical and physiologic changes mediated by thyroid hormone has a dual purpose: (1) to outline some of the changes that may be used as clinical tests in the evaluation of the metabolic status, and (2) to point out the changes in various determinations commonly used in the diagnosis of a variety of nonthyroidal illnesses, which may be affected by the concomitant presence of thyroid hormone deficiency or excess.

Basal Metabolic Rate (BMR)

The BMR has a long history in the evaluation of thyroid function. It measures the oxygen consumption under basal conditions of overnight fast and rest from mental and physical exertion. Since standard equipment for the measurement of BMR may not be readily available, it can be estimated from the oxygen consumed over a timed interval by analysis of samples of expired air.¹⁹⁰ The test indirectly measures metabolic energy expenditure or heat production.

Results are expressed as the percentage of deviation from normal after appropriate corrections have been made for age, sex, and body surface area. Low values are suggestive of hypothyroidism, and high values reflect thyrotoxicosis. The various nonthyroidal illnesses and other factors affecting the BMR, including sources of errors, have been reviewed.¹⁹¹ Although this test is no longer a part of the routine diagnostic armamentarium, it is still useful in research.

Deep Tendon Reflex Relaxation Time (Photomotogram)

Delay in the relaxation time of the deep tendon reflexes, visible to the experienced eye, occurs in hypothyroidism. Several instruments have been devised to quantitate various phases of the Achilles tendon reflex. Although normal values vary according to the phase of the tendon reflex measured, the apparatus used and individual laboratory standards, the approximate adult normal range for the half-relaxation time is 230-390 msec. Diurnal variation, differences with sex, and changes with age, cold exposure, fever, exercise, obesity, and pregnancy have been reported. However, the main reason for the failure of this test as a diagnostic measure of thyroid dysfunction is the large overlap with values obtained in euthyroid patients and alterations caused by nonthyroidal illnesses.¹⁹²

Tests Related to Cardiovascular Function

Thyroid hormone induced changes in the cardiovascular system can be measured by noninvasive techniques. One such test measures the time interval between the onset of the electrocardiographic QRS complex (Q) and the arrival of the pulse wave at the brachial artery, detected by the Korotkoff sound (K) at the antecubital fossa.¹⁹³ Related tests which determine the systolic time interval (STI) measure the preejection period (PEP), obtained by subtraction of the left ventricular ejection time (LVET) from the total electromechanical systole (Q-A2).¹⁹⁴ The left ventricular ejection time (LVET) which is also affected by the thyroid status can be measured by the M mode echocardiogram¹⁹⁵ (Figure 6-5). The PEP/LVET ratio is also useful in the assessment of thyroid hormone action in the cardiovascular system.¹⁹⁶ As with other tests of thyroid hormone action, the principal deficiency of these measurements is their alteration in a variety of nonthyroidal illnesses.

Figure 6-5: Simultaneous tracings of electrocardiogram (ECG), phonocardiogram, carotid pulse and echocardiogram. Measurements of the systolic pre-ejection period (PEP), isovolemic contraction time (ICT), left ventricular ejection time (LVET) and isovolumic relaxation time (IVRT) are indicated. (From I Kline, The thyroid, L.E. Braverman & R.D. Utiger (eds). 1991, J.B. Lippincot Co.)

Miscellaneous Biochemical and Physiologic Changes Related to the Action of Thyroid Hormone on Peripheral Tissues

Thyroid hormone affects the function of a variety of peripheral tissues. Thus, hormone deficiency or excess may alter a number of determinations used in the diagnosis of illnesses unrelated to thyroid hormone dysfunction. Knowledge of the determinations which may be affected by thyroid hormone is important in the interpretation of laboratory data (Table 6-7).