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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Thyroid Disorders in Pregnancy
Sponsored by The Indian Health Service Clinical Support Center
3. Laboratory Testing
If you had only one test you could order to evaluate thyroid function during pregnancy, it would be the free T4. An occasional patient will have hyperthyroidism manifested by a suppressed TSH and a normal free T4, but an elevated free T3, so called “T3-toxicosis”. While this is an uncommon entity, measurement of free T3 may be helpful in the evaluation of the woman who has clinical signs of hyperthyroidism, but has a normal free T4.
Thyroid function testing should always be combined with the clinical evaluation. Iodide levels decrease during pregnancy because of fetal iodine uptake, as well as increased maternal renal clearance of iodine. This may result in a significant increase in the size of the thyroid gland during gestation, which is usually physiologic rather than pathologic. The effect of hCG on this increase in thyroid volume during pregnancy has been alluded to above.
Likewise, the fatigue, weight changes, hyperdynamic circulation, and other normal pregnancy alterations may lead one to suspect thyroid disease. As with everything else in clinical medicine, one needs to put the whole picture together: symptoms, physical findings, and laboratory assessment.
Remember that TSH does not cross the placenta to the fetus, but small amounts of thyroxine do, as do iodine, the various thyroid receptor immunoglobulins (more on them later as well), and the thioamides used to treat hyperthyroidism, propylthiouracil (PTU) and methimazole.
Changes in Thyroid Function Test Results in Normal Pregnancy and in Thyroid Disease
| Status | TSH | FT4 | FTI | TT4 | RT3U | TT3 |
|---|---|---|---|---|---|---|
| Pregnancy | NC/Decr | NC | NC | Incr | Decr | Incr |
| Hyperthyroidism | Decr | Incr | Incr | Incr | Incr | Incr /NC |
| Hypothyroidism | Incr | Decr | Decr | Decr | Decr | Decr/NC |
Abbreviations:
| NC = No change | Inc = Increase |
| Decr = Decrease | TSH = thyroid-stimulating hormone |
| FT4 = free thyroxine | FTI = free thyroxine index |
| TT4 = total thyroxine | TT3 = total triiodothyronine |
| RT3U = resin T3 uptake |
