Mx of a low TSH

see also:

• thyroid physiology and hormones
• hyperthyroidism
• Mx of high TSH
• thyroiditis

• thyroid stimulating hormone (TSH or thyrotropin) is produced by the anterior pituitary gland following stimulation by thyrotropin-releasing hormone (TRH) which is produced from the hypothalamus, and TSH secretion is controlled by a negative feedback loop in response to circulating T3 and T4
• serum TSH is an exquisitely sensitive indicator of thyroid status in patients with an intact hypothalamic pituitary axis
• a low TSH generally signifies hyperthyroidism

• this is usually due to subclinical hyperthyroidism
• other causes include:
  • central hypothyroidism
  • non-thyroidal illness
  • recovery phase from hyperthyroidism
  • acute high dose corticosteroids
  • 1st trimester pregnancy

• most likely to be post-partum thyroiditis which is 10x more common than Grave's disease in this group
• check TSH-receptor antibodies
• consider Tc-99m thyroid scan if not pregnant

• the diagnosis of hyperthyroidism in pregnancy can be challenging.¹⁾
• the diagnosis of hyperthyroidism in pregnancy should be made using serum TSH values, and either total T4 and T3 with total T4 and T3 reference range adjusted at 1.5 times the nonpregnant range or free T4 and free T3 estimations with trimester-specific normal reference ranges.
• serum TSH levels may be below the nonpregnant reference range in the first half of a normal-term pregnancy presumably the result of stimulation of the normal thyroid by high levels of serum hCG
  • therefore, low serum TSH levels with normal free T4 values in early pregnancy do not indicate abnormal thyroid function.
  • transient hCG-mediated thyrotropin suppression in early pregnancy (gestational hyperthyroidism) should not be treated with antithyroid drug therapy.
  • when associated with hyperemesis gravidarum, it is known as transient hyperthyroidism of hyperemesis gravidarum (THHG)
  • it is thought that some people are sensitive to certain isotypes of HCG and this is the cause of this transient hyperthyroidism and may have a direct role in the hyperemesis.
• the most common cause is Grave's disease
• hyperthyroidism caused by a human chorionic gonadotropin (hCG)-producing molar pregnancy or a choriocarcinoma presents with a diffuse hyperactive thyroid similar to GD, but without eye signs and without serum TRAb. In these patients, serum hCG will be higher than expected, and the cause can be identified by obstetrical investigation.
• TRAb levels should be measured when the etiology of hyperthyroidism in pregnancy is uncertain.
• patients found to have GD during pregnancy should have TRAb levels measured at diagnosis using a sensitive assay and, if elevated, again at 22–26 weeks of gestation.
• TRAb levels measured at 22–26 weeks of gestation should be used to guide decisions regarding neonatal monitoring.
• antithyroid drug therapy should be used for hyperthyroidism due to GD that requires treatment during pregnancy. Propylthiouracil should be used when antithyroid drug therapy is started during the first trimester. Carbimazole should be used when antithyroid drug therapy is started after the first trimester.
• when thyroidectomy is necessary for the treatment of hyperthyroidism during pregnancy, the surgery should be performed if possible during the second trimester.
• thyroid scan and radioactive iodide is C/I
• Carbimazole during pregnancy has been associated with birth defects, including aplasia cutis and 'carbimazole embryopathy', characterised by choanal atresia or oesophageal atresia.
• it is usually recommended that propylthiouracil be used in the first trimester and then changed to carbimazole in the second trimester ²⁾
• Thyroid stimulating hormone receptor antibodies are measured during pregnancy as this can predict the risk of neonatal Graves disease

• if there is neck pain:
  • this suggests subacute thyroiditis and there will be near absent uptake on a thyroid scan
• consider Grave's disease features:
  • 5-10x more common in women than men
  • onset peaks at 40-60yrs age
  • may be precipitated by smoking, stress or post-partum period
  • may have the unique features of ophthalmopathy (50% have lid lag, lid retraction or perio-orbital oedema), pretibial myxoedema or clubbing although the latter two are only in 1-2% of cases
• if likely to be Graves disease:
  • commence beta blockers and an antithyroid drug
  • send bloods for TSH receptor antibodies which is the cause of the thyroid hyperstimulation in Graves but up to 10% will have a negative result
  • thyroid scan to confirm homogenous goitre
• if NOT likely to be Grave's disease:
  • send bloods for TSH receptor, thyroglobulin and thyroperoxidase autoantibodies
  • thyroid scan:
    • homogenous uptake ⇒ Grave's disease
    • heterogeneous uptake ⇒ toxic multinodular goitre or toxic adenoma
    • near-absent uptake with a painless neck:
      • painless sporadic thyroiditis
      • drug-associated thyroiditis
      • low thyroglobulin levels:
        • exogenous thyroid hormone:
          • iatrogenic over-replacement
          • intentional TSH suppression
          • factitious ingestion thyroxine