Insulin, Oral Hypoglycemics & Thyroid Drugs

Insulin is the hormone that moves glucose from blood into cells, and injected insulin replaces it when the pancreas fails or cannot keep up. Oral hypoglycemics like metformin lower blood sugar through other routes: reducing liver glucose output, pushing the pancreas to release more insulin, or flushing glucose out in urine. Thyroid drugs correct the opposite problem of hormone excess or shortage: levothyroxine replaces missing thyroid hormone, while carbimazole and propylthiouracil throttle an overactive thyroid.

This matters because India carries one of the world's largest diabetes burdens, and thyroid disorders affect roughly one in ten adults. Insulin dosing errors can cause fatal hypoglycaemia within hours, while thyroid pills taken wrongly simply do not work.

Key Concepts

• Insulin types by speed: rapid (lispro, aspart; onset 15 min), short (regular; 30 min), intermediate (NPH; peaks in hours), long (glargine, detemir; flat 24-hour cover).
• Basal-bolus regimen: long-acting insulin covers background needs; rapid-acting insulin covers each meal, mimicking the healthy pancreas.
• Metformin: first-line for type 2 diabetes; cuts hepatic glucose production, aids modest weight loss; risk of lactic acidosis means it is stopped or reduced in severe kidney failure.
• Other oral agents: sulfonylureas force insulin release but risk hypoglycaemia; DPP-4 inhibitors, SGLT2 inhibitors and GLP-1 agonists act through the gut, kidneys and appetite.
• Levothyroxine: synthetic T4 for hypothyroidism at about 1.6 micrograms per kg per day, taken on an empty stomach 30 to 60 minutes before food; iron and calcium block its absorption.
• Carbimazole and methimazole: block thyroid hormone synthesis in hyperthyroidism; propylthiouracil is preferred in the first trimester of pregnancy.
• TSH, T3, T4: high TSH with low T4 means hypothyroidism; suppressed TSH with high T4 means hyperthyroidism.

Formulas

• Total daily insulin: about 0.5 units per kg per day; roughly half as basal, half divided among meals.
• Carbohydrate ratio (500 rule): 500 divided by total daily dose = grams of carbohydrate covered by 1 unit of rapid insulin.
• Correction factor (1800 rule): 1800 divided by total daily dose = mg/dL fall in blood glucose per 1 unit of rapid insulin.

Worked Example

A 70 kg adult with type 1 diabetes needs a basal-bolus plan. Step 1: total daily dose = 0.5 units/kg times 70 kg = 35 units per day. Step 2: basal = half of 35 = 17.5, rounded to 17 or 18 units of glargine at night. Step 3: the remaining 17 to 18 units split across three meals, about 6 units each. Step 4: carbohydrate ratio = 500 divided by 35 = 14.3, so 1 unit covers roughly 15 g of carbohydrate; a 60 g meal needs 60 divided by 15 = 4 units. Every number traces back to the 0.5 units/kg starting point.

Common Misconceptions

• "Starting insulin means I have failed." Type 2 diabetes is progressive; needing insulin is a stage of the disease, not a personal failure, and it prevents complications.
• "Metformin damages the kidneys." Metformin does not injure kidneys; it is only withheld when kidneys are already severely impaired, because poor clearance raises lactic acidosis risk.
• "I can stop thyroid tablets once TSH is normal." Normal TSH proves the tablet is working, not that the thyroid has healed; stopping lets TSH climb again within weeks.

Real-World Applications

• Type 2 diabetes: metformin first, then add sulfonylurea, SGLT2 inhibitor or GLP-1 agonist stepwise; insulin when targets are missed.
• Graves' hyperthyroidism: carbimazole for 12 to 18 months, propylthiouracil in early pregnancy, radioiodine or surgery for relapse.

Insulin replaces a missing hormone with weight-based basal-bolus math, oral agents attack type 2 diabetes through the liver, pancreas and kidneys, and thyroid drugs either replace T4 or brake an overactive gland. Doses follow formulas