Is it permissible to lie to one's mother about blood sugar levels to avoid her worry and anger, or is it obligatory to tell the truth?

Lying is a forbidden trait, and it is not permissible except in specific cases. It is obligatory to seek truthfulness. Allah Almighty says: ﴿O YOU WHO HAVE BELIEVED, FEAR ALLAH AND BE WITH THE TRUTHFUL.﴾ The Prophet, peace and blessings be upon him, said: "Indeed, truthfulness leads to righteousness, and indeed, righteousness leads to Paradise. And a man continues to be truthful until he is recorded with Allah as a Siddiq (a truthful one). And indeed, lying leads to wickedness, and indeed, wickedness leads to the Fire. And a man continues to lie until he is recorded with Allah as a liar." If informing the mother of the truth pains her, then one should employ tawriyah, which is speech that bears two meanings. For example, regarding a state of high blood sugar, one could say, "Things are good, I am well," or intentionally state a lower blood sugar percentage, or similar subtle expressions (ma'areedh). This tawriyah is used out of necessity, to avoid lying, and it is not proper for it to become a habit for a believer. The Prophet, peace and blessings be upon him, approved of Umm Sulaym's use of ma'areedh when her son passed away, and she said to her husband: "He is more settled than he was." Some imams also did this. So, if the matter is between harmful truthfulness and lying, the way out is to use tawriyah and ma'areedh.