All patients with hypothyroidism are currently treated the same way, regardless of age. The investigators want to look at whether people aged 80 years or older would benefit from being treated with lower doses of levothyroxine. There are three reasons why the investigators think this could be beneficial, but this is not yet proven:

1. Some older people with hypothyroidism may have few symptoms.
2. Doctors look at the amount of Thyroid Stimulating Hormone (TSH) in the patient's blood to decide the dose of Thyroxine received. The standard "normal" TSH range used to determine the dose of levothyroxine is from younger people. The investigators wonder whether this is appropriate to all age ranges particularly as the investigators know that older people may normally have higher TSH values.
3. If TSH levels are too low there may be a slight increased risk of problems such as brittle bones or an irregular heartbeat.

The best way to test whether older people benefit from lower doses of levothyroxine is by a large clinical trial. Before the investigators can do this, the investigators need to run a smaller clinical trial called a "pilot study" (SORTED 1) to examine whether this is practical and acceptable. The pilot study aims to recruit 50 patients with hypothyroidism aged 80 or above.

Participants will be randomly allocated to receive their routine or lower dose of levothyroxine. Follow-up will be conducted over approximately 25 weeks.

The investigators also propose a qualitative study (SORTED 2) to specifically understand patient's willingness to take part in a RCT and participant's experience of the intervention.

Finally, the investigators propose a retrospective cohort study of 400 treated hypothyroid patients aged 80 years or more registered in 2008 in Primary Care Practices with the aim of studying outcomes after 4 years. The cohort study will collect data required to inform a sample size calculation for a future full study where the primary outcome will be 4 year mortality.

Inclusion Criteria:

• Males and Females aged 80 years or older
• Diagnosed with hypothyroidism and treated with Levothyroxine for at least 6 months
• Living independently in the community
• All TSH results within the range 0.4 - 4mU/L in the 3 months before commencing the study
• Participant has provided written informed consent for participation in the study, prior to any study-specific procedures

Exclusion Criteria:

• Established dementia and therefore deemed incapable of providing informed consent.
• Other medical conditions which, inthe opinion of the Chief Investigator, would prevent them from participating in the study (for example, end stage cancer, severe chronic health conditions where the patient is housebound)
• Nursing Homes or Residential Care Home residents
• Individuals with thyroid cancer: since they require high doses of LT4 to suppress their serum TSH
• Individuals on 25 mcg dailty of LT4: dose reduction will mean that they stop thyroid replacement treatment
• Non english speaking individuals
• Participation in any other investigational trials within the last 3 months
• Participants prescribed medications that can affect thyroid function (amiodarone, lithium, carbimazole or propylthiouracil)
• Known or suspected lactose intolerance (this would have implications for the proposed over-encapsulated IMP)