Ronald D. Feld, Ph.D., Marian Schwabbauer, Ph.D., CLDir(NCA)*, John D. Olson, M.D., Ph.D.
Upon completion of this section, the reader will be able to:
1) Describe the basis of urine pregnancy tests.
2) List the limitations of urine pregnancy tests.
3) Identify patients who would benefit from thyroid function screening.
4) Describe the results of thyroid function tests in hyper- and hypothyroidism.
5) List the various tests used to predict ovulation.
Pregnancy Tests
Pregnancy tests are the most common endocrine test performed in the physician office laboratory. Pregnancy tests have progressed analytically from biological assays through agglutination-inhibition to current immunoassays. There has been a concomitant improvement in both sensitivity and specificity with each new analytical procedure.
All pregnancy tests are based on measuring some part of the human chorionic gonadotrophin (HCG) molecule. This hormone is produced by the trophoblast at the time of implantation (about seven days post conception). HCG is part of a family of glycoprotein hormones which includes LH, FSH, and TSH. Each of these molecules contain a common a-subunit and a unique b-subunit to maximize specificity.
Even though these assays are commonly referred to as b-subunit assays, they are for the most part not measuring free b-subunit. In blood, the predominant form of HCG is intact hormone, while in urine, the major form is the beta-core which consists of two polypeptide fragments from the b-subunit joined by a disulphide bridge. The reactive forms of HCG have roughly the same concentration in blood and urine and therefore urine measurements provide equivalent information to blood.
Most urine pregnancy tests come in a "dipstick" form and utilize an enzyme-linked immunosorbent assay (ELISA). This is also known as a sandwich or immunometric assay and utilizes a solid-phase antibody directed to one site on the HCG molecule and the addition of a second labeled antibody directed to another site on the molecule.
The sensitivity of these assays is about 25 IU/L. They will yield positive results 3-4 days after implantation and reach 98% positivity seven days after implantation. This is around the time of the expected menstrual period.
False positive results can occur with levels between 5 IU/L and 25 IU/L and should be repeated at least two days later. HCG may remain elevated for as long as 60 days after a first trimester abortion. HCG secreting tumors will also produce a positive test. Blood contamination from menses and patients with significant proteinuria can also lead to false-positive results. About 3% of ectopic pregnancies have a HCG concentration below 40 IU/L and may be missed. The serum quantitative radioimmunoassay for HCG can measure values between 1-10 IU/L and is therefore considered the gold standard. It can be used when other tests are equivocal.
Negative pregnancy tests can occur before the first missed menses but a negative result at greater than one week after the missed menstrual period is clear evidence of a non-pregnant state. An incorrect menstrual history can, of course, alter the correct interpretation of the test.
Thyroid Tests
Thyroid disease has a high prevalence in certain populations such as the elderly. Clinical symptoms of thyroid disease may be difficult to recognize in this population and in other patients with other illnesses
Aside from screening neonates for congenital hypothyroidism and other populations such as the elderly and patients with autoimmune disease, the routine screening of the general population for thyroid disease is not recommended.
The laboratory diagnosis of thyroid disease has been greatly improved with the introduction of the sensitive TSH assay and either the estimate or measurement of the free T4 (FT4) concentration. The FT4 of an individual is maintained within a narrow range. Linear changes in FT4 cause logarithmic changes in TSH. It is this large response in TSH relative to FT4 that makes it a sensitive test for thyroid disease. As the analytical sensitivity of the TSH assay has improved, the ability to diagnose hyperthyroidism on the basis of a suppressed TSH (> 0.01 mU/L) has also improved. So-called third generation non-isotopic immunometric assays are capable of detecting TSH in the 0.01-0.02 mU/L range.
In diagnosing both hypo- and hyperthyroidism, the first line test is a sensitive TSH assay. In frank hypothyroidism, the TSH should be elevated (> 20 mU/L). The presence of an decreased FT4 helps confirm the diagnosis. The FT4 has the advantage of not being affected by alterations in the affinity or amount of the thyroxine-binding proteins. Subclinical hypothyroidism is characterized by the absence of clinical signs of hypothyroidism, a TSH between 15.0 and 20 mU/L, and a normal or slightly low FT4. Many people with subclinical hypothyroidism convert to frank hypothyroidism over time but the treatment of these individuals is still controversial. Treatment usually consists of exogenous thyroid hormone to normalize the TSH level.
In frank hyperthyroidism, the TSH should be decreased except for TSH secreting pituitary tumors. The majority of hyperthyroidism is caused by Graves' disease. About 90-95% of hyperthyroid patients have TSH concentrations < 0.01 mU/L while a small number have levels between 0.1 and 0.3 mU/L. This disorder is treated by either antithyroid drugs, radioactive iodine therapy, or surgery.
Ovulation Tests
The prediction of ovulation is important in diagnosing reproductive failure and also to determine the optimum time for fertilization. Several properties of cervical mucus such as spinnbarkeit or fibrosity, ferning or crystallization, and viscosity have been used to predict ovulation. The changes in the property of cervical mucus is in response to the hormonal changes during the menstrual cycle.
A rapid rise in luteinizing hormone (LH) occurs 16 to 24 hours prior to ovulation. There is also a lesser peak of follicle stimulating hormone (FSH). There is also a rise in estradiol and other steroids that are synthesized by the developing follicle. The rise in estrogens triggers the LH surge.
The measurement of these hormones in blood can help predict ovulation but the use of blood samples for monitoring is inconvenient and expensive. LH is also found in urine and follows the peak in blood by 6-7 hours.
There are currently enzyme immunoassay tests available for office or home use which measure LH in urine and help predict ovulation. These assays employ two monoclonal antibodies directed to either the a or b chain of LH. One of the antibodies is labeled with an enzyme which caused the color change when substrate is converted to product.
Women with regular menstrual cycles should begin testing for urinary LH about 3 days prior to expected ovulation. In practice the testing period could extend for 7-10 days due to menstrual irregularities. Midday between 1100 to 1400 hours is the best time to measure urinary LH. Once-a-day testing is about 90% accurate while twice-a-day testing at midday and evening increases the accuracy to nearly 100%.