LabMed

Gestational Thyrotoxicosis

At a Glance

Gestational thyrotoxicosis is a rare occurrence during pregnancy and is usually due to Grave's disease (see chapter on Grave’s disease). Presentation is usually in the mid- to late-first trimester. In addition to hypermesis gravidarum, other symptoms are those exhibited in hyperthyroidism: heat intolerance, sweating, angina, tachycardia, nervousness, and moist, warm skin. There can also be enlargement of the thyroid gland.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Measuring serum thyroid stimulating hormone (TSH) concentrations is the most sensitive way to detect abnormalities in thyroid function. This is because a small change in free thyroxine (fT4) results in a logarithmic change in TSH concentrations. In addition, testing for free T4 and free T3 concentrations can also help confirm thyroid abnormalities.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Pregnancy itself alters the normal concentration of thyroid related analytes. As thyroid binding globulin (TBG) concentrations increase, the concentrations of total T4 and total T3 increase and are generally 1.5-fold greater than the upper reference interval of nonpregnant patients. TSH is inversely proportional to hCG; as hCG increases, TSH concentrations decrease, although concentrations are rarely outside the normal reference interval.

Illness can affect thyroid hormone measurements even when the patient has normal thyroid functions. Therefore, it is recommended to not measure thyroid hormones during illness.

A significant number of drugs can alter thyroid function testing. (Table 1)

Table 1.

Thyroid Function Altering Drugs
Drugs Effect on Thyroid Function
dopamine, L-dopa, glucocorticoids, somatostatin decrease T4, decrease T3, decrease TSH
clofibrate, opiates, 5-fluorourazil, perphenazine T4, T3
iodine, lithium decrease T4, decrease T3, TSH
amiodarone, glucocorticoids, propranolol, propylthiouracil decrease T3, decrease or unchanged T4 and FT4, or unchanged TSH
phenobarbital, phenytoin, carbamazepine, rifampicin decrease T4, decrease T3
aluminum hydroxide, ferrous sulfate, iron sucralfate, soybean preparations, kayexalate, cholestyramine, colestipol decrease T4, decrease FT4, TSH
salicyclates, phenytoin, carbamazepine, furosemide,NSAID decrease T4, decrease T3, FT4
androgens, glucocorticoids decrease T4, decrease T3

What Lab Results Are Absolutely Confirmatory?

In thyrotoxicosis, TSH is suppressed (<0.1 mU/L) and free T4 and free T3 are elevated.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Testing for autoantibodies can aid in the diagnosis of thyrotoxicosis. Testing for antithyroid peroxidase (anti-TPO) and anti-TSH receptor antibodies (TSHRAbs) can confirm the diagnosis of Grave's disease. Either antibody is expected to be positive in Grave's disease. Grave's disease can be less severe during pregnancy but can return postpartum. TSHRAbs can cross the placenta and cause transient neonatal Grave's disease by stimulating the fetal thyroid gland. The higher the concentration of TSHRAbs in the third trimester, the higher the risk of acquiring neonatal Grave's disease. A threshold of greater than or equal to 5 Index Units of TSHRAbs has a sensitivity of 100% and specificity of 76% for neonatal thyrotoxicosis.

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