Hypothyroidism and
Pregnancy — the Importance
of TSH Management7

Management of thyroid diseases during pregnancy requires special considerations because pregnancy can induce major changes in thyroid function.

Serum TSH values should be obtained early in pregnancy in the following women at high risk for overt hypothyroidism:

  • History of thyroid dysfunction or prior thyroid surgery
  • Age >30 years
  • Symptoms of thyroid dysfunction or the presence of goiter
  • TPOAb positivity
  • Type 1 diabetes or other autoimmune disorders
  • History of miscarriage or preterm delivery
  • History of head or neck radiation
  • Family history of thyroid dysfunction
  • Morbid obesity (BMI ≥ 40 kg/m2)
  • Use of amiodarone or lithium, or recent administration of iodinated radiologic contrast
  • Infertility
  • Residing in an area of known moderate to severe iodine insufficiency

Recommendation BEFORE Pregnancy

Treated hypothyroid patients (receiving LT4) who are planning pregnancy should have their dose adjusted by their provider in order to optimize serum TSH values.

Recommendation DURING Pregnancy

In pregnant patients with treated hypothyroidism, maternal serum TSH should be monitored approximately every 4 weeks during the first half of pregnancy because further LT4 dose adjustments are often required.

Recommendation AFTER Pregnancy

The necessary LT4 dose adjustments during gestation are a function of pregnancy itself. Therefore, following delivery, LT4 should be reduced to the patient’s preconception dose. Additional TSH testing should be performed at approximately 6 weeks postpartum.

Thyroid Function Test

Thyroid function test results of healthy pregnant women differ from those of healthy nonpregnant women. This calls for pregnancy-specific and ideally trimester-specific reference intervals for all thyroid function tests but in particular for the most widely applied tests, TSH and free T4 (FT4).

If trimester-specific reference ranges for TSH are not available in the laboratory, the following reference ranges are recommended:

First Trimester Second Trimester Third Trimester
0.1–2.5 mIU/L 0.2–3.0 mIU/L 0.3–3.0 mIU/L

Indication and Important Safety Information for Synthroid


  • SYNTHROID® (levothyroxine sodium tablets, USP) is indicated as replacement or supplemental therapy in congenital or acquired hypothyroidism of any etiology, except transient hypothyroidism during the recovery phase of subacute thyroiditis. Specific indications include: primary (thyroidal), secondary (pituitary), and tertiary (hypothalamic) hypothyroidism and subclinical hypothyroidism. Primary hypothyroidism may result from functional deficiency, primary atrophy, partial or total congenital absence of the thyroid gland, or from the effects of surgery, radiation, or drugs, with or without the presence of goiter.

Important Safety Information1


Thyroid hormones, including Synthroid, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss. In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects.

  • Levothyroxine is contraindicated in patients with untreated subclinical or overt thyrotoxicosis, acute myocardial infarction, uncorrected adrenal insufficiency, or with hypersensitivity to any of the inactive tablet ingredients.
  • In patients with nontoxic diffuse goiter or nodular thyroid disease, particularly the elderly or those with underlying cardiovascular disease, levothyroxine sodium therapy is contraindicated if the serum TSH level is already suppressed due to the risk of precipitating overt thyrotoxicosis. If the serum TSH level is not suppressed, levothyroxine should be used with caution in conjunction with careful monitoring of thyroid function for evidence of hyperthyroidism and clinical monitoring for adverse cardiovascular signs and symptoms of hyperthyroidism.
  • Levothyroxine should not be used in the treatment of male or female infertility unless this condition is associated with hypothyroidism.
  • Levothyroxine has a narrow therapeutic index. Regardless of the indication for use, careful dosage titration is necessary to avoid the consequences of over- or under-treatment.
  • In women, long-term levothyroxine sodium therapy has been associated with increased bone resorption, thereby decreasing bone mineral density, especially in post-menopausal women on greater than replacement doses or in women who are receiving suppressive doses of levothyroxine sodium.
  • Patients receiving levothyroxine sodium should be given the minimum dose necessary to achieve the desired response.
  • Patients with coronary artery disease who are receiving levothyroxine therapy should be monitored closely during surgical procedures, since the possibility of precipitating cardiac arrhythmias may be greater in those treated with levothyroxine.
  • In patients with secondary or tertiary hypothyroidism, additional hypothalamic/pituitary hormone deficiencies should be considered, and, if diagnosed, treated.
  • Patients with concomitant adrenal insufficiency should be treated with replacement glucocorticoids prior to initiation of treatment with levothyroxine sodium. Failure to do so may precipitate an acute adrenal crisis when thyroid hormone therapy is initiated. Patients with diabetes mellitus may require upward adjustments of their antidiabetic therapeutic regimens when treated with levothyroxine.
  • Adverse reactions associated with levothyroxine therapy are primarily those of hyperthyroidism due to therapeutic overdosage.
  • Levothyroxine should not be discontinued during pregnancy and hypothyroidism diagnosed during pregnancy should be promptly treated.
  • Drug Interactions: Many drugs affect thyroid hormone pharmacokinetics and metabolism, and thyroid hormones and thyroid status have varied effects on the pharmacokinetics and actions of other drugs. Levothyroxine increases the response to oral anticoagulant therapy and may reduce the therapeutic effects of digitalis glycosides. Prescribers should consult appropriate reference sources for drug-thyroidal axis interactions.