Comparison of various thyroid function tests
Huang, H.S.; Pittman, C.S.
Journal of the Formosan Medical Association 84(11): 1251-1259
1985
ISSN/ISBN: 0371-7682 PMID: 3868700 Document Number: 247609
To compare the responsiveness of various thyroid hormone tissue parameters to thyroid hormone replacement, 10 subjects were studied, 6 females and 4 males, with mean age of 38 .+-. 16 years (Mean .+-. S.D.), 8 with differentiated thyroid cancer, 2 with Graves' tyrotoxicosis. They were hypothyroid after thyroidectomy and/or radioactive iodine treatment. All were replaced with levothyroxine 0.1-0.2 mg daily prior to this study. Serum thyroid hormones and thyroid hormone tissue parameters were studied at 2-weekly intervals for 4-6 weeks while thyroxine replacement was discontinued and then at weekly intervals for 4 weeks after thyroxine therapy was re-instituted. Measurement of thyroid hormone tissue parameters included cardiac systolic time intervals, e.g. preejection period (PEP) and left ventricular ejection time (LVET), basal metabolic rate (BMR), ankle jerk reflex by photomotogram, left ventricular shortening fraction (LVSF), peripheral nerve conduction velocity (NCV) and fasting blood for glucose, GOT, .gamma.-glutamyl transferase (.gamma.-GT), creatine phosphokinase (CPK), lactate dehydrogenase (LDH), alkaline phosphatase, cholesterol and triglyceride. When listed by order of decreasing correlation with levels of circulating thyroid hormones, the parameters may be ranked as PEP, PEP/LVET, BMR, photomotogram, alkaline phosphatase, cholesterol and LVSF. Nerve conduction velocity showed no relation to thyroid medication or circulating thyroid hormone. Serum level of T3, T4, free T4, free T3, TSH, showed an order of increasing correlation with a given hormone effect. No significant correlation could be obtained between fasting blood sugar, triglyceride, LDH, GOT, .gamma.-GT, CPK and serum level of thyroid hormones. Upon thyroid replacement, serum T4 and T3 normalized first (average 9 days) followed by cholesterol, PEP, CPK, photomotogram, BMR, PEP/LVET and TSH. TSH was the last parameter to reduce to normal (average 24 days). When levothyroxine was discontinued, T4 and TSH became abnormal first (average 2 weeks), followed by PEP, photomotogram, BMR, PEP/LVET, T3, CPK and cholesterol. Cholesterol was the last one to become abnormal (average 1 month). The results indicate that (1) cardiac systolic time interval-PEP, is the most sensitive measurement available for thyroid hormone tissue effect, (2) serum TSH is best correlated with thyroid hormone tissue effect, (3) nerve conduction velocity is the least sensitive indicator of thyroid hormone tissue effect, (4) it takes 24 days for hypothyroid patients to become euthyroid in all metabolic indices after thyroid hormone replacement, however, it takes one month to become hypothyroid after the withdrawal of thyroid hormone therapy.