A blood specimen for neonatal thyroid screening is obtained on –
Neonatal thyroid screening: Guidelines by Indian Academy of Pediatrics 

Screening should ideally be done for all newborns: because there are generally not signs or symptoms of hypothyroidism at birth.
Use cord blood, or wait for 5-7 days for sampling: results are difficult to interpret in between.
Test for T4 and TSH : insist on an early report (within 2-3 days). level of TSH upto 10 piu/ml is considered normal upto 8-10 weeks of age.
If abnormal, repeat to confirm diagnosis.
If conveniently possible WITHOUT DELAY, do thyroid scan before starting treatment.
Explain the problem, and the consequences of not treating, in detail to parents to ensure compliance.
Begin treatment with thyroxin (not thyroid extract) as early as possible: dose 10-12 mg/kg/day as single daily dose.
Monitor with TSH levels : at age of 2 weeks, then every 2 months for the first year of life, and then every 3-6 months till puberty is completed.
Soon after beginning therapy, monitor T4 also for rapid assessment of the adequacy of dose. After the first year, in a good laboratory, T4 assessment, alone should be adequate for monitoring.
Review at age of 2-3 years to reconfirm diagnosis. Thyroxine should be stopped for 6-8 weeks and T4, TSH repeated. Thyroid scan should be done at this time if not done before starting treatment
Cord blood
24 hours after birth
48 hours after birth
72 hours after birth