Pregnancy Complicated by Thyroid Conditions

December 14, 2007

The Thyroid Gland and Pregnancy

Simply put, the thyroid gland is like a governor on a motor it sets the speed of metabolism for your body.  If it sets that metabolism too high, you can burn up your motor, so to speak.  Weight loss, rapid heart rate, and a host of other unpleasant side effects can alter the way you feel with a higher than normal metabolism.  In fact, when morning sickness of pregnancy increases in intensify into the uglier hyperemesis, an abnormality of the thyroid function should be suspected.

  • Euthyroid: is a medical slang word meaning everything is just fine with this gland.
  • Hyperthyroid: means increased functioning into the abnormal range, as described above.
  • Hypothyroid: means decreased function, with resulting weight gain (you don't burn up the calories), lethargy and fatigue, and possibly goiter formation (enlargement of a lackluster thyroid that tries to overgrow to make up for the decreased function).

Like most glands, there are hormones related to the thyroid and its function.  The thyroid cranks out mainly T3 and T4 (tri-iodothyronine and thyroxine, respectively), and calcitonin.  The T3 and T4 are what are called the thyroid hormones, and they differ from each other in that the T3 is more powerful while the T4 is more plentiful.  They're related to each other in that T4 is converted into the more potent T3 in the blood stream. Calcitonin helps manage calcium levels in the body, which is important in bone health, since so much of bone is calcium.

The Hypothalamic-Pituitary-Thyroid Axis

TSH, "thyroid stimulating hormone," is made by the pituitary gland.  The pituitary gland gets signals from another gland even higher up in the pecking order, the hypothalamus.  The ultimate output of T3 and T4 from the thyroid gland is based on readings of what's in the circulation already, such that hypofunction (under-function) prompts the stimulating hormones to prod some extra action from the thyroid, and hyperfunction (over-function) results in a blunting of the output of these stimulating hormones.  This whole back-and-forth is like a thermostat, and it's a trick that is used throughout the body on many levels.  (For instance, the entire menstrual cycle works like this via an hypothalamus-pituitary-ovarian axis.)

The Thyroid Hormones and Pregnancy

Properly stimulated by the pituitary and hypothalamus (the vice president and the CEO of the body, respectively), the thyroid cranks out the T3 and T4 as needed.  These thyroid hormones are bound to proteins which render them inactive until they're freed-then they can do their job.

Both are made using Iodine, so it's important to supplement the diet (iodized salt, for instance) with Iodine in areas where it isn't plentiful.

What they do

>The thyroid hormones adjust the metabolism (how fast you "run") by stimulating enzymes that take part in sugar and energy use, temperature regulation, and the burning of calories.  The intestinal function (movement and absorption) is fine-tuned for the purpose of altering sugar absorption.

They help with the management of fat accumulation or breakdown, which is associated with temperature regulation.

Thyroid hormones affect the cardiovascular system both directly and indirectly. Cholesterol elimination is controlled by thyroid hormones, influencing the tendency toward or away from heart disease.  The functional pumping of the heart is directly stimulated by thyroid hormones, resulting in palpitations when there is too much thyroid stimulation (as in hyperthyroidism).

Growth and puberty onset are controlled by the thyroid.  Thyroid hormones help control bone growth by affecting the breakdown of calcium.  They promote the manufacture of proteins, which affect growth and function of the muscles in the body.

Thyroid hormones take part in the development and functioning of the breasts.  Milk production from the breast in the non-pregnant woman ("galactorrhea") should prompt thyroid testing in addition to the other usual tests.

The adrenal gland (which is a whole different story with its own pantheon of hormones) is stimulated by the thyroid.  This indirectly can affect steroid production (like adrenalin).

One of the ways temperature regulation is also controlled is by control of sweat glands.  The skin's oil glands are also directed by thyroid hormone, and skin texture can sometimes be a tip-off to a thyroid condition.

Almost a third of patients with depression have thyroid problems, and it's not yet known which causes which.  For this reason, depression is another condition that warrants thyroid testing.  Normal thyroid function is necessary for normal fetal development of the brain, with both hypothyroidism and hyperthyroidism significant risk factor for intellectual impairment.

Calcitonin is a thyroid hormone that helps maintain the level of calcium in the blood stream (helps deposit it into bone formation, thereby lowering the calcium levels in the blood).  This hormone works in concert with the other thyroid effects related to calcium and bone growth.

When things go wrong

Autoimmune disorders in which antibodies attack the thyroid tissue, can cause increased stimulation as well as destruction of the thyroid (hypothyroidism or hyperthyroidism).

Hypothyroidism and Pregnancy

Decreased functioning of the thyroid can be because of Iodine deficiency (not enough Iodine to make the hormones) or because of destruction of the thyroid gland (radiation or auto-immune disease).  Being from the New Orleans area, I have Iodine comin' out my ears, thanks to the seafood in my diet.  But there are places where supplemental Iodine needs to be considered.  Autoimmune disease can produce an inflammation ("thyroiditis") that can limit function.  Removal of the thyroid, necessary to remove thyroid tumors, can remove enough functioning thyroid to cause hypothyroidism, too.

In pregnancy, there is increased excretion of Iodine, plus the fetus sifts a lot of it out of the maternal circulation, so that if a woman has an Iodine deficiency to begin with, there is an increased danger of hypothyroidism.  Usually prenatal vitamins solve this dietary problem, though.

Symptoms of hypothyroidism are lethargy, cold intolerance, weight gain, and hair and skin changes.  In its most severe form, called "myxedema," there is excessive sedation, abnormally low sodium levels, respiratory distress, and possibly death.  Myxedema is rare, usually seen in undiagnosed elderly patients, making pregnancy myxedema a non-issue.

Treatment of hypothyroidism is usually by way of supplementing a patient with T4 (e.g., Synthroid).

Out of control or undiagnosed hypothyroidism can complicate pregnancy by causing cretinism (growth and brain abnormalities) in the fetus if the maternal hypothyroidism is severe enough to be associated with Iodine deficiency in the fetus, too.  But proper control of hypothyroidism should be plenty good enough to prevent these types of problems. 

Treatment is based on the cause, treated with either a prescription of levothyroxine (T4, as the drug Synthroid) or Iodine supplementation, or both.  Synthroid is FDA Drug Class B, so it's considered safe, especially since not treating hypothyroidism carries the risks above.

Hyperthyroidism and Pregnancy

Abnormally increased functioning of the thyroid is usually due to Graves' Disease.  In this autoimmune disorder, there is over-stimulation, resulting in hyperthyroidism.  Another cause can be an over-functioning "hot spot"-called a toxic nodule (of thyroid tissue).  And if you really want to get into some weird stuff, there can be thyroid tissue in a type of tumor of the ovary (dermoid cyst), which can be functional enough to cause hyperthyroidism ("struma ovarii").  I've even had a case of thyroid cancer arising from such ovarian tissue, but this is extremely rare.

Symptoms of hyperthyroidism include restlessness, fast pulse, intolerance to heat, and weight loss.  The bulging eyes, called "exopthalmos," are the most striking physical characteristic.  How bad can it get?  In a word...very.  "Thyroid storm" is a name that's as bad as it sounds.  It can result in fever, rapid pulse to the point of atrial fibrillation, shock, confusion, psychosis, seizures, coma, and death.

In pregnancy, hyperthyroidism can aggravate morning sickness into the much more debilitating hyperemesis gravidarum, requiring hospitalization for rehydration.  (In my practice, anyone with morning sickness is evaluated for thyroid function again after the initial labs.)  But all this is just talking about the mother so far, but hyperthyroidism can impact the pregnancy, too, with increased risk toward preterm labor, premature deliveries, low birth weight infants, and pregnancy induced hypertension (pre-eclampsia).  The antibodies that make the mother's thyroid overreact can pass through the placenta to the baby, causing hyperthyroidism in the fetus, too.

Because pregnancy tones down a woman's immune response to everything (so that she won't reject her baby), immune disease-caused hyperthyroidism may cool down after a temporary surge of activity at 15 weeks, but there isn't agreement about the studies that have suggested this.

But based on this, besides the postpartum time for a patient, her doctor will be wary of "thyrotoxicosis" in the early to mid 2nd trimester as well.

Treatment of hyperthyroidism in pregnancy is a little more challenging than treating hypothyroidism (where merely supplementing T4 may be enough).  Medicines to tone down thyroid function are a bit more artificial than the more natural T4-like Synthroid.  Since these are chemically constructed drugs, prescribing them may walk a patient down the FDA letters of risk (T4 is only "B," little if any risk).

PTU(propylthiouracil) is the time-honored therapy.  This drug works by interfering with the thyroid gland's use of Iodine to make thyroid hormones. Unfortunately, PTU crosses the placenta to the baby and can cause a mild hypothyroid condition-even goiters-in a minority of newborns.  For this reason, it's FDA risk letter is "D."  The risk to the fetus is late in pregnancy, because the fetal thyroid does not begin making thyroid hormones until the end of the first trimester.  Still, it's considered the safest approach to hyperthyroidism.  Not treating hyperthyroidism is even riskier.

Steroids are used to help control thyroid storm and have a significant safety margin in pregnancy.  Regardless, thyroid storm is extremely dangerous and can't go untreated.

Thyroid Confusion: Hyperemesis Gravidarum-Morning Sickness

Hyperemesis gravidarum is the abnormal distortion to the more common morning sickness.  Weight loss is never normal in pregnancy, and nutrition is crucial during that first trimester when major developmental trends are happening in the fetus.  So when the nausea and vomiting progresses into an unstoppable starvation, hospitalization is necessary.

At a molecular level, there is a similarity of the pregnancy hormone hCG to the thyroid hormone.  They both sport an identical component-the "alpha chain molecule." (It's the "beta-chain molecule" on each that makes them the different hormones that they are.)  Unfortunately, the rise in hCG that comes with pregnancy, and with it a rise in an alpha-chain molecule, makes the body react like there's too much thyroid hormone, aggravating a patient into a morning sickness or even hyperemesis. No one knows if the alpha-chain of hCG, making the body act like there's too much thyroid hormone, is THE reason for morning sickness, but certainly there's some input from this phenomenon.  In the absence of actual hyperthyroidism, hyperemesis is treated with hydration and anti-nausea medications, not with medicines to treat hyperthyroidism.

In Summary--Thyroid Complications

Autoimmune diseases can make the thyroid hyperfunction or hypofunction.  Thyroid hormone supplementation treats "hypo," and "hyper" is treated with drugs like PTU that interfere with using iodine to make thyroid hormones.  Both conditions are dangerous to mother, baby, and the pregnancy on the whole if untreated.

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