The TSH (Thyroid Stimulating Hormone) test measures the level of TSH, a hormone produced by the anterior pituitary gland, which regulates the synthesis and release of thyroid hormones — Thyroxine (T₄) and Triiodothyronine (T₃). TSH levels are very sensitive indicators of thyroid gland function, making this test the first-line screening tool for detecting hypothyroidism and hyperthyroidism. The prime function of TSH is to regulate the synthesis and secretion of the thyroid hormones viz. T3 and T4. The determination of TSH serves as the initial test in thyroid diagnostics. Alteration in the TSH level indicates either hyperthyroidism ( low TSH level ) or hypothyroidism (high TSH). However, only TSH determination would not help in disease diagnosis. It should always be performed along with Free T3 and Free T4 for a proper clinical diagnosis. Various research studies have indicated considerable levels of biological and analytical variations in TSH measurement. It may be attributed to mainly due to the pulsatile secretion of the hormone and fairly short half life ( 1 - 2 hrs). Research studies have indicated that circadian variation (i.e variation of morning and evening samples) of as high as 50% variation. This is more so even in case of pregnant ladies. Hence TSH interpretation should be both diagnostic and clinical, not a single factor alone.