Digestive glands are the glands that produce and release various substances that aid in the digestion process. They secrete enzymes, hormones, and other digestive juices that help break down food and absorb nutrients. The liver, pancreas, and salivary glands are all considered digestive glands because they play a crucial role in digestion. However, the thyroid gland is not involved in digestion and therefore is not considered a digestive gland.

**Liver:**
- The liver is the largest internal organ in the human body and is located in the upper right portion of the abdomen.
- It produces bile, a substance that helps break down fats in the small intestine during digestion.
- Bile is stored in the gallbladder and released into the small intestine when needed.
- The liver also metabolizes nutrients, detoxifies harmful substances, stores vitamins and minerals, and produces blood-clotting proteins.

**Pancreas:**
- The pancreas is a glandular organ located behind the stomach and connects to the small intestine.
- It produces digestive enzymes that are released into the small intestine to break down carbohydrates, proteins, and fats.
- The pancreas also produces insulin and glucagon, hormones that regulate blood sugar levels.

**Salivary glands:**
- Salivary glands are located in the mouth and produce saliva.
- Saliva contains enzymes, such as amylase, which begin the digestion of carbohydrates.
- Saliva also lubricates the food, making it easier to swallow and aiding in the initial stages of digestion.

**Thyroid gland:**
- The thyroid gland is an endocrine gland located in the neck, below the Adam's apple.
- It is responsible for producing hormones that regulate metabolism, growth, and development.
- The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), primarily control the body's metabolic rate.
- While the thyroid gland plays a crucial role in overall body function, it is not directly involved in the digestion process.

Therefore, the correct answer is option 'D' - Thyroid gland, as it is not a digestive gland.

Thyroid gland help in secretions of thyroxin hormone . Iodine is necessary for metabolism of thyroxin hormone . deficiency cause hypothyroidism and goitre.