**Core Concept**
Lithium is a commonly used mood stabilizer in the management of bipolar disorder. However, its use in pregnancy is a concern due to potential teratogenic effects. Monitoring for lithium levels and renal function is crucial in pregnant females.

**Why the Correct Answer is Right**
Lithium crosses the placenta and can cause fetal goiter and hypothyroidism. The primary abnormality to check for in a pregnant female taking lithium is an increase in thyroid-stimulating hormone (TSH) levels, indicating hypothyroidism. This is due to the inhibition of thyroid hormone release by lithium, leading to a compensatory increase in TSH production.

**Why Each Wrong Option is Incorrect**
**Option A:** Electrolyte imbalance, while a concern with lithium, is not the primary abnormality to check for in pregnancy.
**Option B:** Renal function, although monitored with lithium, is not the primary abnormality to check for in pregnancy.
**Option C:** Cardiac arrhythmias are not directly related to lithium use in pregnancy.

**Clinical Pearl / High-Yield Fact**
It's essential to monitor thyroid function tests (TFTs) in pregnant females taking lithium, as hypothyroidism can have significant consequences for both mother and fetus.

**Correct Answer: C. Cardiac arrhythmias are not directly related to lithium use in pregnancy.