Thyroid in Pregnancy

Thyroid disease

Physiological changes in pregnancy Hepatic synthesis of thyroid binding globulin is increased

Total levels of thyroxin and tri iodothyronine are increased to compensate for this rise

Level of free T4 are altered less by pregnancy but do fall a little in the second and third trimesters

Serum concentration of thyroid stimulating hormone initially rise and then fall in the first trimester and the normal range is wider than in the non pregnant .

Hyperemesis gravidarum may be associated with a biochemical hyper thryroidism with high levels of free T4 and a a suppressed TSH  p to 60% of cases. This relates to increased concentration of human chorionic gonadotrophin . hCG has thyrotropic activity .

In the second and third trimesters. TSH levels increase so the upper limit of the reference rqange is raised compared with those in the non pregnant woman

Similarly the normal ranges for free T4 and T3 are reduced . Compared to outside pregnancy free T4 has a narrower and lower range and falls throughout pregnancy.

TSH levels used in isolation are unreliable in pregnancy for the assessment of thyroid status.

Pregnancy is associated with as state of relative iodine deficiency that has two major cases:

1.       Maternal iodine requirements increase because of active transport to the fetoplacental unit.

2.       2. Iodine excretion in the urine is increased twofold because of increased glomerular filtration and decreased renal tubular reabsorption.

3.       Because the plasma level of iodine falls the thyroid gland increases its uptake from the blood threefold.

If there is already dietary insufficiency of iodine the thyroid gland hypertrophies in order to trap a sufficient amount of iodine.

Pregnancy specific normal ranges or TFTs

	TSH	Thyroxine	Tri iodothyronine
Non- pregnant	0.27-4.2	12-22	3.1-6.8
First trimester	0-5.5	10-16	3-7
Second trimester	0.5-3.5	9-15.5	3-5.5
Third trimester	0.5-4	8-14.5	2.5-5.5

Biochemical assessment of thyroid function in pregnancy should include assays of free T4 and in   some cases free T3 . Immunoradiometric assays of TSH are useful but should not be used in isolation because of the variable effects of gestation.

Pattern of abnormality	Possible diagnoses	Comments/further investigation versus normal non-pregnant ranges in women
Total T4 Total T3 Normal free T4 Normal TSH	Normal in pregnancy	Refer to normal   ranges for pregnancy
Free T4 (mild) TSH (mild)	Normal in third trimester Mild hypothyroidism	Refer to normal ranges for third trimester Check thyroid autoantibodies
Normal free T4 TSH	May be normal feature in early first trimester May represent sub clinical hypothyroidism possibly with poor compliance	Repeat thyroid function tests in second trimester check thyroid autoantibodies TSH may remain high in the initial phases of treatment of hypothyroidism
Free T4 TSH	May be associated with hyperemesis in the absence of nausea or vomiting or in association with other symptoms preceding pregnancy or thyroid eye disease suggests thyrotoxicosis	Does not require treatment if due to hyperemesis Abnormality resolves with improvement in hyperemesis Check thyroid stimulating antibodies to help confirm diagnosis of thyrotoxicosis and assess risk of fetal hyperthyroidism
TSH Free T4	Secondary or tertiary hypothyroidism or non thyroidal illness	Both secondary and tertiary hypothyroidism are rare
Normal free T4	Treated thyrotoxicosis possibly with an intermittently compliant patient May be a normal feature in first trimester	TSH remains suppressed in the initial phases of treatment of hyperthyroidism Repeat thyroid function tests in second trimester.