Overt hypothyroidism is defined as a raised TSH level in association with a decreased level of free T4.

Subclinical hypothyroidism is diagnosed when the TSH level is elevated, but T4 is still within the normal range.

Overt hypothyroidism is estimated in 0.3–0.5 % of women, and subclinical hypothyroidism in 2–3 % of pregnant women.

The thyroid gland has an important function in maintaining a viable pregnancy and in contributing to the development of a healthy offspring.

There are increased requirements for T4 in pregnancy, and the foetus is totally dependent on the placenta to provide thyroxine until about 18 weeks gestation.

The structure of HCG is similar to TSH and provides some stimulation to T4 production. This results in TSH being suppressed by the negative feedback, eventually resulting in a decrease of T4.

In general, iodine deficiency is the commonest cause of thyroid insufficiency worldwide. The second commonest cause is auto-immune thyroiditis (Hashimotos’s disease).