The goal of the current study was to evaluate the relationship between haematological, blood metabolic, and lactation phases in the critically endangered Teressa goat (TG) of the Andaman and Nicobar Islands (ANI). From the goat breeding facility of the ICAR-Central Island Agricultural Research Institute (CIARI), Port Blair, ANI, 30 healthy Teressa goats aged 3–4 years were chosen. From day 1 to 135 of lactation, 15-day interval was used to examine the haematological, serum biochemical, antioxidant, and endocrinological profiles, which were then split into the first (day 1 to 45), second (day 46 to 90), and third (day 91 to 135) stages of lactation. The amount of milk produced on average (in mL) varied greatly depending on the stage of lactation (first stage: 763.17 ± 17.37; second stage: 506.58 ± 18.79; third stage: 226.13 ± 15.59). Between lactational phases, haematological profiles did not alter much, although they did fall within the range of caprine species. While albumin, aspartate amino transferase (AST), and alanine amino transferase (ALT) decreased significantly as lactation stages advanced, serum biochemical profiles such as glucose, total cholesterol (TC), total protein (TP), globulin, calcium, phosphorous, and magnesium increased significantly as lactation stage advanced. There were no statistically significant differences in TG, ALP, or creatinine levels among lactational phases. Similarly, serum triiodothyronine (T3) and thyroxine (T4) increased, prolactin and cortisol markedly decreased as lactation phases advanced. As the stage advanced, the total antioxidant capacity (TAC) rose and the oxidative stress marker malondialdehyde (MDA) considerably reduced. In the humid tropical island ecology of ANI, it is established that the first and second lactational stages in endangered Teressa goats are more stressful than the third stage.